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Abstract

Cannabis has been used for centuries to treat seizures. Recent anecdotal reports, accumulating animal model data, and mechanistic insights have raised interest in cannabis-based antiepileptic therapies. In this study, we review current understanding of the endocannabinoid system, characterize the pro- and anticonvulsive effects of cannabinoids [e.g., Δ9-tetrahydrocannabinol and cannabidiol (CBD)], and highlight scientific evidence from pre-clinical and clinical trials of cannabinoids in epilepsy. These studies suggest that CBD avoids the psychoactive effects of the endocannabinoid system to provide a well-tolerated, promising therapeutic for the treatment of seizures, while whole-plant cannabis can both contribute to and reduce seizures. Finally, we discuss results from a new multicenter, open-label study using CBD in a population with treatment-resistant epilepsy. In all, we seek to evaluate our current understanding of cannabinoids in epilepsy and guide future basic science and clinical studies.
REVIEW
Cannabinoids and Epilepsy
Evan C. Rosenberg
1
&Richard W. Tsien
1
&Benjamin J. Whalley
2
&Orrin Devinsky
3
#The American Society for Experimental NeuroTherapeutics, Inc. 2015
Abstract Cannabis has been used for centuries to treat sei-
zures. Recent anecdotal reports, accumulating animal model
data, and mechanistic insights have raised interest in cannabis-
based antiepileptic therapies. In this study, we review current
understanding of the endocannabinoid system, characterize
the pro- and anticonvulsive effects of cannabinoids [e.g.,
Δ9-tetrahydrocannabinol and cannabidiol (CBD)], and high-
light scientific evidence from pre-clinical and clinical trials of
cannabinoids in epilepsy. These studies suggest that CBD
avoids the psychoactive effects of the endocannabinoid
system to provide a well-tolerated, promising therapeutic for
the treatment of seizures, while whole-plant cannabis can both
contribute to and reduce seizures. Finally, we discuss results
from a new multicenter, open-label study using CBD in a
population with treatment-resistant epilepsy. In all, we seek
to evaluate our current understanding of cannabinoids in
epilepsy and guide future basic science and clinical studies.
Keywords Epilepsy .seizures .cannabinoids .cannabidiol .
THC .cannabis
Introduction
Epilepsy affects 2.9 million people in the USA and 65 million
people worldwide (cdc.gov/epilepsy). One in 26 people in the
USA will develop epilepsy in their lifetime [1]. Characterized
by recurrent seizures, epilepsy encompasses multiple disor-
ders caused by varied etiologies, including genetic syndromes,
stroke, infection, and traumatic brain injury. Many patients
with epilepsy also have sensorimotor, cognitive, psychologi-
cal, psychiatric, and social impairments, as well as impaired
quality of life and an increased risk of premature death [1].
While epilepsy can affect patients of all ages, it most com-
monly affects children, the elderly, and individuals with low
socioeconomic status. The estimated direct and indirect annu-
al cost of epilepsy in the U.S. is $15.5 billion (cdc.gov/
epilepsy).
While many drugs can limit seizures, no drug can prevent
the underlying cause of epilepsy or the development of epi-
lepsy (epileptogenesis) in patients who are at risk (e.g., after
head trauma). A third of patients remain pharmacoresistant,
failing to achieve sustained seizure freedom after 2 or more
adequately chosen, tolerated, and appropriately used antiepi-
leptic drugs (AEDs; more accurately termed antiseizure drugs)
[24]. Patients resistant to multiple AEDs have an increased
risk for sudden unexpected death in epilepsy and other forms
of epilepsy-related mortality [5,6], as well as impairments in
psychosocial, behavioral, and cognitive functions [3,79].
For many patients, epilepsy is a progressive disorder associ-
ated with ongoing loss of brain tissue and function. Finally,
multidrug combinations and high dosages cause more severe
side effects, a particular problem in patients with treatment-
resistant epilepsies. Assessing the side effects of AEDs is es-
pecially challenging in patients on long-term AEDs as any
baselinemay be many years past and even intelligent adults,
parents, and physicians may fail to appreciate chronic adverse
Electronic supplementary material The online version of this article
(doi:10.1007/s13311-015-0375-5) contains supplementary material,
which is available to authorized users.
*Orrin Devinsky
od4@nyu.edu
1
Department of Neuroscience and Physiology, Neuroscience Institute,
NYU Langone Medical Center, New York, NY 10016, USA
2
School of Pharmacy, The University of Reading, Whiteknights,
Reading RG6 6AP, UK
3
Department of Neurology, Comprehensive Epilepsy Center, New
York University School of Medicine, New York, NY 10016, UK
Neurotherapeutics
DOI 10.1007/s13311-015-0375-5
effects. The available AEDs fail to meet the clinical needs for
both efficacy and safety [10], indicating a dire need for novel
therapeutics that are targeted, disease-, and age-specific.
Recently, mounting anecdotal reports and media coverage
have sparked intense interest among parents, patients, and the
scientific community regarding the potential of medical can-
nabis to treat seizures. A potential alternative or supplement to
current AEDs, the cannabis plant includes >100 diverse
phytocannabinoids that, in part, target an endogenous
endocannabinoid signaling network, as well as other net-
works. Two major phytocannabinoids derived from cannabis
are psychoactive Δ9- tetrahydrocannabinol (THC) and
nonpsychoactive cannabidiol (CBD). Both Δ9-THC and
CBD can prevent seizures and reduce mortality in animal
models of seizure with low toxicity and high tolerability
[11]. However, a systematic analysis from the American
Academy of Neurology and a Cochrane Database review both
concluded that medical cannabis is of unknown efficacyto
treat epilepsy [12,13]. In this review, we examine the history
of cannabinoids in epilepsy, discuss the effectiveness of pre
clinical seizure model studies with cannabinoids, and review
recent clinical data, including a multicenter clinical trial of
CBD for patients with treatment-resistant epilepsy.
History of Cannabis in Epilepsy
Cannabis has been used for millennia for medical, recreation-
al, and manufacturing purposes. Around 2900 BCE, the Chi-
nese Emperor Fu Hsi characterized cannabis as having sacred
yin (weak, passive forces) and yang (strong, active forces)
features, suggesting that it could restore homeostasis to an
unbalanced body. Physicians in ancient India, Egypt, Persia,
Rome, Arabia, and Greece used cannabis for spiritual and
medicinal purposes, including menstrual fatigue, gout, rheu-
matism, malaria, beriberi, constipation, pain, and absentmind-
edness [14]. Early documented uses of cannabis to treat sei-
zures include a Sumerian text from 2900 BCE and an Arabian
document from the twelfth century [15,16].
The 1854, the US Dispensatory listed cannabis to treat
neuralgia,depression, pain,muscle spasms, insomnia, tetanus,
chorea, insanity, and other disorders [17]. Cannabis was val-
ued for its analgesic, anti-inflammatory, appetite-stimulating,
and antibiotic properties. In the mid-1800s, the British sur-
geon William OShaughnessy reported cannabis therapy for
the treatment of epilepsy, recounting an alleviation of pain in
most, a remarkable increase of appetite in all, unequivocal
aphrodisia, and great mental cheerfulness[14,18]. Two of
Englands most prominent mid-to-late nineteenth- century
neurologists, J.R. Reynolds and W. Gowers, also noted the
benefits of cannabis in epilepsy [19]. Gowers reported a man
whopreviouslyfailedbromideswhoseseizureswere
controlled on 9.8 g of Cannabis indica, dosed 3 times daily
for up to 6 months [20].
Cannabis was first regulated in the USA with the 1906
Pure Food and Drug Act. The follow-up 1937 Marijuana
Tax Act was opposed by the American Medical Association,
which considered the more severe restrictions an infringement
on physiciansfreedomtotreatpatients[17]. In 1970, the US
Comprehensive Drug Abuse Prevention and Control Act cat-
egorized marijuana as a Schedule I drug with high potential
for abuse and no accepted medicinal use. Legislation has been
introduced to the US Senate to change marijuana to a Sched-
ule II drug.
Over the last 50 years, the main chemical constituents of
cannabis have been isolated and synthesized. Δ9-THC was
isolated in 1964 and synthesized in 1971 [21,22]. CBD was
isolated in 1940 and synthesized in 1963 [23,24]. The canna-
binoid type 1 (CB
1
R) and type 2 (CB
2
R) receptors, which
bind Δ9-THC, were cloned in the 1990s [25,26], supporting
an endogenous system for this principal cannabinoidsphar-
macological activity.
The Endocannabinoid System
The discovery of the endocannabinoid system in the early
1990s revealed the neuronal mechanisms that underlie the
psychoactive effects of Δ9-THC in cannabis. Initial studies
demonstrated that brief postsynaptic depolarization re-
duced neurotransmitter release from excitatory terminals
onto Purkinje cells in the cerebellum and inhibitory termi-
nals onto pyramidal neurons in the hippocampus [27,28].
This phenomenon was termed depolarization-induced
suppression of excitation/inhibition(DSE and DSI, re-
spectively). Postsynaptic depolarization was postulated to
trigger the release of an undiscovered substance that tran-
siently limited presynaptic neurotransmitter release. Along
with the discovery of nitric oxide (NO), this paradigm-
shifting view suggested the concept of retrograde signaling
in contrast to a primarily anterograde view of synaptic sig-
naling. Application of a CB
1
R agonist (or antagonist) en-
hanced (or prevented) DSE and DSI, suggesting that it was
mediated by an endogenous cannabinoid ligand [2931].
These endocannabinoids were identified as the hydropho-
bic ligands N-arachidonoyl ethanolamide (anandamide)
[32] and 2-arachidonoyl glycerol (2-AG) [33,34].
Anandamide and 2-AG are synthesized from postsynaptic
membrane phospholipid precursors and released in an activi-
ty-dependent, on-demandmanner, unlike traditional vesic-
ular neurotransmitters (Fig. 1). Depolarization of the postsyn-
aptic cell or direct activation of metabotropic glutamate recep-
tors increases levels of intracellular calcium, which trigger
second messenger cascades that promote endocannabinoid
synthesis [3539]. Anandamide is synthesized via
phospholipase D-mediated hydrolysis of N-arachidonoyl-
phosphatidylethanolamine, and degraded by the fatty acid am-
ide hydrolase (FAAH) into arachidonic acid and ethanolamine
[4043]. 2-AG is synthesized via diacylglycerol (DAG) lipase
(DAGL) α-mediated hydrolysis of DAG, and degraded by
FAAH into arachidonic acid and glycerol, or by
monoacylglycerol lipase [4144]. Chronic hyperexcitability
leads to dynamic changes in the endocannabinoid pathway
(see The Endocannabinoid System:CB
1
Rs). Thus, the en-
zymes that regulate metabolism and cannabinoid receptors rep-
resent attractive targets to treat several neurological disorders
[45]. Accordingly, the selective CB
1
R blocker rimonabant was
approved in >50 countries as an anorectic to treat obesity [46],
and showed promise in helping smokers quit tobacco use [47],
but its use was suspended when postmarketing surveillance
revealed high rates of depression and suicidal ideation.
Produced in an activity-dependent manner, endocannabinoids
travel to the presynaptic cell and bind to CB
1
Rs. CB
1
Rs are G
protein-coupled receptors linked to pertussis-sensitive Gi/o α
subunits. Activation of the αsubunit triggers dissociation of
the βγ complex, which reduces adenylate cyclase production
of cyclic adenosine monophosphate [48], inhibits N- and P/Q-
type voltage-gated calcium channels [31,4952], stimulates A-
type potassium channels [5356], activates G protein-coupled
inwardly-rectifying potassium channels [5759], and inhibits
the vesicular release machinery [60]. These multiple mechanisms
reduce presynaptic cell excitability and Ca
2+
, strongly
diminishing presynaptic neurotransmitter release. CB
1
Rs can al-
so regulate the presynaptic release of multiple neuromodulators
such as acetylcholine, dopamine, and norepinephrine [61]. Final-
ly, endocannabinoid signaling may modulate regional-specific
long-term synaptic plasticity, including long-term potentiation
and long-term depression (for a review, see [62,63]).
CB
1
Rs are distributed primarily in axon terminals in the
neocortex (especially cingulate, frontal, and parietal re-
gions), hippocampus, amygdala, basal ganglia, thalamus,
hypothalamus, nucleus accumbens, substantia nigra, ven-
tral tegmental area, cerebellum, and brainstem [39]. CB
1
Rs
are most densely expressed at cortical and hippocampal
presynaptic γ-aminobutyric acid (GABA)ergic presynaptic
boutons, especially cholecystokinin-positive (CCK+) and
parvalbumin-negative GABAergic interneurons [6466].
Glutamatergic axon terminals in cortical and subcortical
neurons contain fewer presynaptic CB1 receptors than
GABAergic terminals [65,6771].
Phytocannabinoids: Classification and Function
The cannabis plant consists contains >100 C21 terpenophenolic
compounds, known collectively as phytocannabinoids [72].
Most of these lipophilic cannabinoids are closely related and
differ only by a single chemical functional group. Cannabinoids
fall into 10 main groups, with constituents representing degra-
dation products, precursors, or byproducts (Fig. 2, adapted from
[73]). Two of the most abundant constituents are Δ9-THC and
CBD, the ratios of which vary by cannabis strain. Cannabis
sativa contains a higher ratio of Δ9-THC to CBD, producing
more stimulating, psychotropic effects. Cannabis indica strains
contains a higher ratio of CBD:Δ9-THC and are typically more
sedating [11,73].
Δ9-THC
Δ9-THC is a partial agonist at central nervous system (CNS)
CB
1
Rs and CB
2
Rs in the immune system. Most behavioral,
cognitive, and psychotropic effects of cannabis result from the
effects of Δ9-THC at brain CB
1
Rs. The subjective high
produced by cannabis can be blocked by pretreatment with
the CB
1
R antagonist rimonabant [74]. Δ9-THC impairs
short-term working memory in several rodent models, which
canbereversedbypreapplicationofaCB
1
R antagonist
[7578]. Inhibition of long-term potentiation at hippocampal
CA3 Schaffer Collateral/CA1 synapses may underlie this
a
b
Fig. 1 Biosynthesis, degradation, and signaling of endocannabinoids. (A)
Presynaptic cannabinoid type 1 receptor (CB
1
R) signaling. (B) Postsynaptic
endocannabinoid biosynthesis/signaling. NArPE = N-arachidonoyl
phosphatidylethanolamine; DAG = 1-acyl, 2-arachidonoyl diacylglycerol;
VGCC = voltage-gated calcium channels; PEA = palmitoylethanolamide;
ACPA = arachidonylcyclopropylamide; ACEA = arachidonyl-2'-
chloroethylamide; PMSF = phenylmethylsulfonyl fluoride
Cannabinoids and Epilepsy
effect on memory [35]. Δ9-THC or CB
1
R agonists can in-
crease or decrease food intake in different species [79]. Δ9-
THC also regulates neuronal excitability during seizures (see
Preclinical Evidence). Thus, Δ9-THC acts through the
endocannabinoid system toregulate mood, learning and mem-
ory, neuronal excitability, and energy balance. Δ9-THC exerts
potent anti-inflammatory functions via CB
1
Rs and CB
2
Rs on
microglia, the primary immune cells in the CNS. Δ9-THC or
CB
1
R agonists limit neurotoxicity in in vitro and in vivo as-
says, including chemotoxic [8083], low Mg
2+
[84], and is-
chemic [85,86]models.Δ9-THC has antioxidant effects in α-
amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid- and
N-methyl-D-aspartate-mediated cytotoxicity models via a
CB
1
R-independent mechanism [87]. Cannabinoids reduce
neuronal and glial release of the proinflammatory cytokines
tumor necrosis factor-α, NO, interleukin (IL)-1 and IL-6
[8893], and increase release of anti-inflammatory cytokines
IL-4, IL-10, and IL-1 receptor antagonist (IL-1a) [94,95]via
CB1R- and CB2R-dependent mechanisms in neurons and glia
[94,95] (reviewed in [96]). Δ9-THC also transiently activates
and desensitizes the transient receptor potential (TRP) chan-
nels TRPA1, TRPV1, and TRPV2 [9799]. Given the syner-
gistic relation between seizures and inflammation [100102],
the cannabinoid system provides a novel strategy to target
both segments of this feedback cycle.
CBD
CBD resembles Δ9-THC structurally but the 2 molecules
differ significantly in pharmacology and function. CBD has
very low affinity at CB
1
RandCB
2
R, unlike Δ9-THC
[103106]. The potential targets for CBD are reviewed in
detail in another article in this issue (Molecular Targets of
CBD in Neurological Disorders). CBD is an agonist at TRP
channels (TRPV1, TRPV2, TRPA1) [98,99,104], 5-
hydroxytryptamine1αreceptors [107], and glycine receptors
[108]. CBD is an antagonist at TRP melastatin type-8 chan-
nels [97], T-type voltage-gated calcium channels [109], and G
protein-coupled-receptor GPR55 (see below).
CBD exerts dynamic control over intracellular calcium
stores through multiple, activity-dependent pathways [110,
111]. CBD induces a bidirectional change in intracellular cal-
cium levels that depends on cellular excitability. Under normal
physiological Ca
2+
conditions, CBD slightly increases intra-
cellular Ca
2+
, whereas CBD reduces intracellular Ca
2+
under
high-excitability conditions. These changes were blocked by
the pretreatment with an antagonist of the mitochondrial Na
+
/
Ca
2+
exchanger, suggesting a mitochondrial site of action
[111]. CBD also produces biphasic changes in intracellular
calcium levels via antagonism of the mitochondrial voltage-
dependent anion channel 1 [112].
CBD antagonizes GPR55, which functions as a counterpart
to the canonical CB1R/CB2R signaling pathway [113].
GPR55 is present in the caudate, putamen, hippocampus, thal-
amus, pons, cerebellum, frontal cortex, and thalamus. GPR55
was initially characterized as a novel cannabinoid receptor,
coupled to Gα13 [114]. Activation of GPR55 in human em-
bryonic kidney cells triggers the release of intracellular Ca
2+
from endoplasmic reticulum stores via a pathway dependent
on RhoA (Ras homolog gene family member A), phospholi-
pase C, and inositol 1,4.5-trisphosphate receptor [115]. The
endogenous membrane phospholipid L-α-
lysophosphatidylinositol is a GPR55 agonist [116]. Brief ap-
plication of L-α-lysophosphatidylinositol transiently in-
creases intracellular Ca
2+
levels and vesicular release proba-
bility at excitatory hippocampal synapses. CBD opposes this
effect by reducing glutamate release, suggesting a potential
antiseizure mechanism [117]. CBD also reduces epileptiform
activity (burst amplitude and duration) in in vitro (4-
aminopyridine and Mg
2+
) models through a CB1-indepen-
dent, concentration-dependent, and region-specific manner
in the hippocampus. Preclinical studies demonstrate an anti-
seizure effect of CBD (see Preclinical Evidence).
CBD also regulates several transporters, enzymes, and met-
abolic pathways that are common to Δ9-THC and
endocannabinoid signaling. CBD inhibits uptake of adenosine
by blocking the equilibrative nucleoside transporter [118,
119]. Increased levels of adenosine activate A2 receptors,
which regulate striatal CB
1
Rs [120]. At high micromolar
Fig. 2 Biosynthesis of phytocannabinoids [73]
levels, CBD also inhibits the uptake and enzymatic degrada-
tion of anandamide via FAAH, elevating anandamide extra-
cellular concentrations [121]. Thus, dynamic interactions like-
ly occur between the multiple plant cannabinoids such as
CBD and Δ9-THC (see Entourage Effect).
CBD limits inflammation and oxidative stress [122]. CBD
reduces oxidative toxicity in an in vitro glutamate excitotoxicity
assay [123], and raises adenosine to oppose lipopolysaccharide-
induced inflammation and tumor necrosis factor-αrelease [118,
124]. In mice with middle cerebral artery occlusion, CBD trig-
gered a CB
1
R-independent decrease in reperfusion injury, in-
flammation, and death. This neuroprotective action may result
from reduced myeloperoxidase activity, neutrophil migration,
and microglia high-mobility group box 1 expression [125,
126]. Additionally, CBD activates peroxisome proliferator-
activated receptor-γ, reduces NO and IL-βrelease, limits gliosis,
and restricts neuroinflammation in mice injected with amyloid β
[127129]. Finally, treatment of microglial cultures with
interferon-γraised mRNA levels of the CBD receptor GPR55
[130], which regulates the inflammatory responses to neuropath-
ic pain [131]. Taken together, these studies suggest that CBD
reduces neuroinflammation in several disease-specific
conditions.
The Entourage Effect
The entourage effectwas a term originally coined by Ben-
Shabat et al. [132] to refer to the potentiating effects of
endocannabinoid metabolic byproducts on endocannabinoid
function at CB
1
Rs and CB
2
Rs. They observed that 2 esters of
the endocannabinoid 2-AGs2-linoleoyl-glycerol and 2-
palmitoyl-glycerolwere present in spleen, brain, and gut, to-
gether with 2-AG. While these esters do not bind to cannabi-
noid receptors or inhibit adenylyl cyclase via either CB
1
or
CB
2
, each ester potentiated 2-AG-induced inhibition of motor
behavior, immobility on a ring, analgesia on a hot plate, and
hypothermia: behavioral tests commonly referred to as the tet-
radby which CB
1
-mediated effects can be detected [132].
Thus, the original concept of the entourage effect referred to a
specific group of endogenous compounds, structurally similar
to endocannabinoids, that potentiated the effects of endogenous
cannabinoid receptor agonists at CB
1
Rs and CB
2
Rs.
Subsequently, the idea of theentourage effect has expanded
considerably both with regard to mechanisms of interactions,
as well as classes of chemical agents. The diversification of
entourage effects has been promoted by scientific and lay
authors, and often well beyond its original boundaries. Wag-
ner and Ulrich-Merzenich [133] proposed 4 potential mecha-
nisms of synergy for phytotherapeutics, using cannabis as an
exemplar: 1) multitarget effects; 2) pharmacokinetic effects
(e.g., improved bioavailability or solubility); 3) improved bac-
terial resistance; and 4) modulation of adverse events (AEs;
truly an antagonism, albeit a beneficial one) [133]. This ap-
proach thereby extended the tightly defined entourage effect
to include practically any plant mixture acting through any
molecular target to exert any effect.
The cannabis plant contains a complex mixture of both
cannabinoids (i.e., Δ9-THC and CBD) and terpenoids
(limonene, myrcene, α-pinene, linalool, β-caryophyllene,
caryophyllene oxide, nerolidol, and phytol) derived from a
common precursor (geranyl pyrophosphate). Several stud-
ies posited that the entourageof [whole] plants are bet-
ter drugs than the natural products isolated from them,
suggesting that the clinical effects of cannabis usage may
be due to complex interactions between several plant can-
nabinoids [134,135]. In support of this view, CBD may
potentiate the beneficial effects associated with Δ9-THC
(analgesia, antiemesis, and anti-inflammation) and reduce
the negative psychoactive effects of Δ9-THC (impaired
working memory, sedation, tachycardia, and paranoia)
[136138]. Users of cannabis with a high CBD:Δ9-THC
ratio have greater tolerability and lower rates of psychosis
than users of high Δ9-THC:CBD ratios (or Δ9-THC
alone) [139]. Additional reports claim potential synergis-
tic interactions of phytocannabinoids and phytoterpenoids
that may include therapeutic effects on pain, inflamma-
tion, depression, anxiety, addiction, epilepsy, cancer, fun-
gal, and bacterial infections [135,140]. However, proper
characterization of any synergisticeffects of multiple
plant cannabinoids requires statistically robust demonstra-
tions of effects greater than the sum of the parts. These
effects can be tested in vitro or in vivo using assays such
as the isobolographic approach [141,142]. Such a design
can show if any 2 compounds, extracts, or mixtures are
additive in the specific assay (e.g., models of seizure),
synergistic, or antagonistic, thereby avoiding speculation
about potential synergism or the confusion of additive
effects with synergism. Although experimental data sup-
port the efficacy of both CBD and Δ9-THC as individ-
ual agents in various animal models of epilepsy, we are
not aware of any studies demonstrating synergy of these
compounds in animal models nor any controlled trials
that establish a synergistic effect in patients with
epilepsy.
Collectively, several studies demonstrate functional (but
not defined molecular) interactions between plant cannabi-
noids that extended the initial concept of the entourage effect
far beyond its original intent. While such interactions may
exist, further well-defined research is required to verify anec-
dotal claims regarding the increased antiseizure efficacy of
CBD with Δ9-THC (vs CBD alone) in patients with epilepsy.
While natural selection may have led to combinations of phy-
tochemicals in cannabis to resist infection or predation, there
is no reason to expect natureto combine chemicals in a
single plant to treat human epilepsy.
Cannabinoids and Epilepsy
Animal Models of Seizures and Epilepsy
Animal models provide powerful assays to assess the po-
tential antiseizure or antiepileptic effects of cannabinoids.
Each preclinical paradigm has unique advantages and dis-
advantages, and many represent unique seizure etiologies,
semiologies, or corresponding electroencephalography
(EEG) patterns. Table 1(adapted from [143145]) summa-
rizes animal models discussed in this review, grouped by
relevance to human epilepsies. Acute models (e.g., kainic
acid and pentylenetetrazol) allow high-throughput screen-
ing for upregulation of biomarkers, but cannot recapitulate
spontaneous recurrent seizures or reduced seizure thresh-
olds found in chronic epilepsy. Chronic models of seizure
activity elicit spontaneous, recurrent seizures that can be
recorded on video EEG. While technically challenging, the-
se models better represent epileptogenesis and drug screen-
ing for humans. However chronic models are specific to the
type of insult (traumatic brain injury, mouse genetic
models), and may not reflect broad anatomical or functional
changes in generalized epilepsy [145].
Preclinical Evidence of Cannabinoids in Epilepsy
Multiple animal models demonstrate the efficacy of cannabi-
noids in preventing seizures and reducing mortality in epilep-
sy. Animal models highlight dynamic changes in the
endocannabinoid system follow chronic seizures, with both
acute and chronic homeostatic regulatory components.
The Endocannabinoid System
Endocannabinoid release prevents seizure-induced neurotoxic-
ity. Kainic acid (KA) (30 mg/kg)-induced seizures increased
levels of the anandamide in wild-type mice (20 min postinjec-
tion) [146], and pilocarpine (375 mg/kg)-induced seizures in-
creased levels of 2-AG (15 min postseizure onset) [147]. Thus,
epileptiform activity triggers a neuroprotective, on-demand re-
lease of endocannabinoids (or increase endocannabinoid levels
in a downstream pathway unrelated to neuroprotection). Pre-
treatment with an anandamide reuptake inhibitor (UCM707;
3 mg/kg) reduced KA-induced seizure severity, but not in mice
with conditional CB1R deletion in principal forebrain excitato-
ry neurons [146]. Blockade of the endocannabinoid catabolic
enzyme FAAH (with AM374; 8 mg/kg) increased levels of
anandamide and protected against KA (10 mg/kg)-induced hip-
pocampal seizures and subsequent impairments in balance and
coordination [148]. Inhibition of both FAAH (with AM374)
and the anandamide reuptake transporter (with AM404) in rat
hippocampus prevented α-amino-3-hydroxy-5-methyl-4-
isoxazolepropionic acid-induced excitotoxic insults (cytoskele-
tal damage and synaptic decline) in vitro and behavioral and
memory impairment in vivo [149]. Blockage of FAAH and
DAGLα(with AM6701, 5 mg/kg) raised levels of anandamide
and 2-AG, protected against KA (10 mg/kg)-induced seizures,
and reduced seizure-induced cytotoxicity [150]. The
endocannabinoids, methanandamide, and 2-AG reduced neu-
ronal firing in a low Mg
2+
in vitro model of status epilepticus, in
a dose-dependent manner (EC
50
145±4.15 nM
methanandamide, 1.68± 0.19 μM 2-AG) [151].
CB
1
Rs
Animal models demonstrate that activation of CB
1
Rs reduces
seizure severity. Mice with conditional deletion of CB
1
Rs in
principal forebrain excitatory neurons (but not interneurons)
exhibited more severe KA-induced seizures (30 mg/kg) than
wild-type controls. Conditional deletion of the CB
1
Rin-
creased gliosis and apoptosis following KA-induced seizures
and prevented activation of the protective immediate early
genes (c-Fos, Zif268, brain-derived neurotrophic factor)
[146]. CB
1
R expression in hippocampal glutamatergic (but
not GABAergic) inputs is necessary and sufficient to protect
against KA-induced seizures [152]. Further, viral-induced
overexpression of CB
1
Rs targeted to the hippocampus re-
duced KA-induced seizure severity, seizure-induced CA3 py-
ramidal cell death, and mortality [153], Together, these results
demonstrate that CB
1
Rs could limit seizure activity and pro-
tect neurons from subsequent cell death and reactive gliosis.
Seizures trigger homeostatic changes in hippocampal
CB
1
Rs and the endocannabinoid system (reviewed in [154])
(Fig. 3). Levels of CB
1
R expression in the CA1-3 stratum
oriens and radiatum (presumed excitatory inputs) and dentate
gyrus steadily increased 1-week post-pilocarpine-induced sei-
zures (Fig. 3, dark green trace) [147,155158]. However,
sclerotic and nonsclerotic hippocampal tissue resected from
patients with epilepsy displayed a reduction in DAGLα(2-
AG biosynthetic enzyme), CB
1
R mRNA, and CB
1
R excitato-
ry terminal immunoreactivity (Fig. 3, light green trace) [159].
Furthermore, compared with healthy controls, patients with
temporal lobe epilepsy have reduced levels of anandamide
in cerebrospinal fluid samples [160]. These findings suggest
that seizure activity induces a homeostatic upregulation of
excitatory terminal CB
1
Rs, which may reduce excitatory neu-
rotransmitter release via DSE (see The Endocannabinoid
System). This compensatory process may be impaired in
patients with prolonged treatment-resistant epilepsy or hippo-
campal sclerosis, leading to neuronal hyperexcitability,
pharmacoresistance, and inconsistent effects of cannabis ex-
posure. However, further research is required to verify the
functional effects of this potential process in human patients,
and whether CB
1
R homeostasis indeed limits seizure severity
or occurrence.
In contrast to effects at excitatory terminals, seizures induce a
homeostatic reduction in CB
1
R expression in inhibitory
Tab le 1 Preclinical animal models of seizures (adapted from [143145])
Type of seizure model Method Mechanism Relevant human condition Common use
Acute MES Electrical stimulation Generalized tonicclonic
seizure
Drug screening (used as a first-line
screening method for AEDs)
Acute PTZ GABA
A
R antagonist, Ca
2+
channels
(?), Na
+
channels (?)
Generalized seizure Drug screening (used as a first-line
screening method for AEDs),
seizure mechanism
Acute KA Ionotropic glutamate receptor (e.g.,
AMPAR, kainate receptor agonist)
Focal (temporal lobe) seizure Drug screening, mechanism of seizures/
epileptogenesis and cognitive impairments
Acute Flurothyl GABA
A
R antagonist Multiple acute seizures,
childhood epilepsy
Development of cognitive impairments
from early life seizures
Acute Other chemoconvulsant
(e.g., bicuculline, 3-MPA,
picrotoxin, etc.)
Various Generalized seizures (or focal,
if applied locally)
Drug screening
Acute Hypoxia/ischemia Anoxic depolarization, impaired Na
+
/K
+
ATPa se , extracellular impairments K
+
/
[glutamte]/[aspartate], intracellular
Na
+
,Ca
2+
Hypoxicischemic encephalopathy Age-specific (e.g., neonatal) drug screening,
mechanisms of seizures and cognitive
impairments
Acute Hyperthermia Activation of temperature-sensitive ion
channels, release of proinflammatory
cytokines
Febrile seizures Drug screening, long-term consequences
of seizures
Chronic with high
propensity for induced
seizures
Lithium/pilocarpine-induced
chemical kindling
AChR agonist Focal (temporal lobe) seizures Drug screening, mechanism of seizures/
epileptogenesis and cognitive impairments
Chronic with high
propensity for induced
seizures
Electrical (e.g., 6Hz
psychomotor, limbic)
kindling
Electric stimulation Focal (temporal lobe) seizures Drug screening, mechanism of seizures/
epileptogenesis and cognitive impairments
Chronic epilepsy (SRS) Stroke, TBI Disease-specific models Focal epilepsy Drug screening, mechanism of seizures/
epileptogenesis and cognitive impairments
Chronic epilepsy (SRS) SE Chronic treatment with KA or
pilocarpine
Prolonged seizures Drug screening
Chronic epilepsy (SRS) Genetic (e.g., GAERs,
WAG/Ij mice,
photosensitive baboons)
Various Specific seizure models (e.g.,
absence seizures, genetic)
Drug screening
SRS = spontaneously recurring seizures; MES = maximal electroshock; PTZ = pentylenetetrazole; KA = kainic acid; 3-MPA = 3-mercaptopropionic acid; TBI = traumatic brain injury; SE = status
epilepticus; GAERs = genetic absence epilepsy rats from Strousberg; GABA
A
R=γ-aminobutyric acid type A receptor; AMPAR = α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor;
ATPase = adenosine triphosphatase; AChR = acetylcholine receptor; AEDs = antiepileptic drugs
Cannabinoids and Epilepsy
terminals (Fig. 3, dark red trace). Beginning 4 days following
pilocarpine-induced seizures in rats, CB
1
R expression progres-
sively decreased in hippocampal CCK+ inhibitory nerve termi-
nals [161], particularly in the CA1 stratum pyramidale and the
dentate gyrus inner molecular layer, unaccounted for by CA1
neuronal cell loss alone [155,156,162]. By reducing CB
1
R
expression on inhibitory terminals (and presumed DSI), this
homoeostatic process may limit network disinhibtion and restrain
elevated excitability during prolonged epileptiform activity. In
sclerotic hippocampal tissue removed from 1) 2 months post
pilocarpine-induced seizures in mice [163], and 2) human pa-
tients [164], levels of CB
1
Rs remained consistently elevated in
interneuron axonal terminals (Fig. 3, light red trace). This finding
suggests that patients with prolonged, pharmacoresistant epilepsy
may suffer from impaired CB
1
R homeostasis on inhibitory inter-
neuron terminals, leading to prolonged disinhibition and network
excitability. Postseizure changes in CB
1
Rs may be specific to
seizure type or developmental stage, as mice with a single epi-
sode of febrile seizures induce an overall increase in DSI and
CB
1
R on CCK+ interneurons [165,166].
Modulators of the Endocannabinoid System
and Synthetic CB
1
R Agonists/Antagonists
Figure 4summarizes the effects of synthetic cannabinoids and
phytocannabinoids in 175 pre-clinical seizure models or dis-
crete conditions (adapted from [167]). These studies are
subclassified by drug typeand seizure model in corresponding
tables in the Appendix (see Supplementary Material).
Results from 13 studies from 3 species (rat, mouse, guinea
pig) demonstrate that modulation of the endocannabinoid sys-
tem (via inhibition of FAAH or anandamide reuptake) pro-
vides about 46.2 % (6/13) anticonvulsant, 23.1 % (3/13)
mixed effect, and 30.8 % (4/13) no significant effect in seizure
models. CB
1
R agonists produced an anticonvulsant effect in
68.1 % (47/69), proconvulsant effect in 2.9 % (2/69), mixed
effect in 7.2 % (5/69), and no significant effect in 21.7 % (15/
69) of seizure models in rats and mice. One study suggests that
CB
1
R agonists may produce an anticonvulsant effect through
CB
1
Rs at low doses, but a proconvulsive effect through
TRPV1 channels at high doses [168]. In addition, CB
1
Rago-
nists (WIN55, 212, ACEA) often produce a additive effect
when combined with several commonly prescribed AEDs
(see Fig. 4B)[169177]. In 18 studies from mice, rats, and
guinea pigs, CB
1
R antagonists were pro convulsant in 38.9 %
(7/18), anti-convulsant in 5.6 % (1/18), and showed no signif-
icant effect in 55.6 % (10/18) of trials. Although CB
1
Rago-
nists were anticonvulsant in 68.1 % of the studies, only 38.9 %
of CB
1
R antagonists were proconvulsive (most showed no
effect). Thus, while activation of the endocannabinoid system
may prevent long-term consequences of seizure sequelae, in-
hibition of the endogenous protective mechanisms may not
contribute significantly to seizures. Variations in the pro- vs
anticonvulsant effects in each system may reflect specific ef-
fects of the species, seizure models (acute vs chronic, focal vs
generalized), dose ranges, timing, or experimental design.
Phytocannabinoids: Δ9-THC and CBD
Evidence from 34 studies from 6 animal species demonstrate
that Δ9-THC is anticonvulsant in 61.8 % (21/34),
proconvulsant in 2.9 % (1/34), mixed in 2.9 % (1/34), and
shows no significant effect in 32.4 % (11/34) of seizure
models. Δ9-THC potentiated the effects of phenytoin and
phenobarbital in the maximal electroshock model of general-
ized seizures [178,179]. The National Toxicology Program
noted a pro convulsant effect of Δ9-THC in rats and mice
[180], although species-specific differences in CB
1
Rexpres-
sion may underlie variable responses to Δ9-THC. CBD and
its homologue cannabidivarin (CBDV) were 80.5 % (33/41)
anticonvulsive and 19.5 % (8/41) ineffective, at reducing sei-
zures in mice and rats. Notably, no studies showed a
proconvulsive effect for CBD or CBDV. CBDV potentiated
the effects of phenobarbital, ethosuximide, and valproate in 2
seizure models [181]. These studies suggest that both Δ9-
THC and CBD provide significant protection from seizures
Fig. 3 Homeostatic changes to
hippocampal cannabinoid type 1
receptors (CB
1
Rs) in preclinical
animal seizure models [147,
154166]. GABA = γ-
aminobutryic acid
in preclinical animal trials, presenting potential targets for hu-
man studies.
Tole ra nc e a nd W ithdr aw al
Prolonged treatment with Δ9-THC or synthetic CB1 agonists
leads to a dose-dependent and region-specific desensitization,
downregulation, and internalization of CB
1
Rs [182203].
These changes produce tolerance to the acute behavioral ef-
fects of Δ9-THC in in vivo models, reducing cannabinoid-
induced hypomotility, hypothermia, antinocioception, and
memory impairment with repeated usage [182,183,185,
197,199,204,205]. In several seizure models, prolonged
Δ9-THC (but not CBD) exposure leads to tolerance to the
antiepileptic activity of cannabinoids [206210]. In humans,
chronic cannabinoid usage produces tolerance towards Δ9-
THC-mediated changes in autonomic behaviors, sleep and
sleep EEGs, and self-reported psychotropic high, although
these changes vary in frequent users [211216].
Withdrawal from rats chronically dosed with Δ9-THC trig-
gers rebound seizures and elevated anxiety-like responses in
several preclinical animal studies [217219]. Monkeys that
previously self-administered intravenous Δ9-THC demon-
strate abstinent symptoms of aggressiveness, hyperirritability,
and anorexia [220], as well as impaired operant behavior
[221]. Results from human studies demonstrated symptoms
of anxiety, aggression, dysphoria, irritability, anorexia, sleep
disturbances, and sweating during abstinence from chronic
Δ9-THC usage, rescued by Δ9-THC re-administration
[222]. Withdrawal from cannabis use can trigger rebound sei-
zures in several preclinical animal and human studies [203,
209,210,223226], although other studies show no
proconvulsant effect of cannabis withdrawal [178,227].
Unlike Δ9-THC, CBD (or nabiximols, CBD/Δ9-THC in a
1:1 ratio) does not seem to produce significant intoxication [228],
tolerance [229231], or withdrawal effects [232]. CBD and/or
nabiximols may counteract the Δ9-THC-dominant effects of
cannabis withdrawal [233235]. In summary, evidence suggests
that while both tolerance and some withdrawal symptoms may
occur with Δ9-THC, CBD may limit the effects of cannabis
tolerance and withdrawal, but more studies are needed.
Clinical Evidence of Cannabinoids in Epilepsy
Several clinical studies have examined the association be-
tween cannabis use and seizures. These include case studies,
surveys and epidemiological studies, and clinical trials.
Case Studies
Case reports describe proconvulsant and anticonvulsant effect
of cannabis, with the majority reporting either beneficial or
lack of effect on seizure control. Selected examples illustrate
the diverse spectrum of reported responses. Cannabis used 7
times within 3 weeks was associated withmultiple tonicclon-
ic seizures in a patient previously seizure free for 6 months on
phenytoin and phenobarbital. However, seizures were not
temporally correlated with immediate intoxication or with-
drawal [236]. Cannabis withdrawal increased complex partial
seizure frequency in a 29-year-old man with a history of alco-
holism and bipolar disorder (each of with are independently
associated with seizures) [226]. In another 2-part case study, a
43-year-old on carbamazepine experienced about 56nightly
violent seizures lasting 1 min each. When he consumed about
40 mg C. sativa at night, seizure frequency was reduced by
70 %, but withdrawal triggered a doubling of his baseline
seizure frequency. In the same study, a 60-year-old man with
a 40-year history of cannabis usage (68 cigarettes per day)
developed status epilepticus after cannabis withdrawal [225].
Additionally, synthetic designercannabinoid drugs (spice
or K2) induce new-onset seizures, tacharrythmia, and psy-
chosis, often with greater severity and toxicity than cannabis
[237245]. The toxicity of these synthetic agents may result
from their properties as full agonists of CB
1
R, while Δ9-THC
is a partial agonist.
The majority of other studies demonstrate an anticonvul-
sant effect of cannabis. In a 1949 trial, administration of a Δ9-
THC homolog (1,2-dimethyl heptyl) reduced the severe an-
ticonvulsant resistant (phenobarbital or phenytoin) grand mal
epilepsyin 2/5 children [246]. One patient whose seizures
were not controlled on low-dose phenobarbital or phenytoin
had fewer tonicclonic seizures while smoking 25cannabis
cigarettes per day [247]. Myoclonic and other seizures were
reportedly reduced in 3 adolescents on oral 0.070.14 mg/kg
Δ9-THC daily. Parents reported that their children were
more relaxedmore alert, more interested in her surround-
ings[248]. In another study, a 45-year-old man with cerebral
palsy and treatment-resistant focal epilepsy experienced a
marked reduction in focal and secondary generalized seizures
on daily marijuana [249]. Other recent cases also support the
observation that cannabis use can reduce seizures in some
patients [250,251]. These studies suggest that cannabis can
not only reduce seizure susceptibility, but also trigger rebound
seizures during withdrawal. Limitations of open-label, often
retrospective single case reports are compounded by the var-
iability in epilepsy syndrome, differences in cannabis dosage,
route, and composition.
Epidemiological Reports and Surveys
Recent epidemiological reports and surveys depict the incidence
of medical marijuana usage for seizure control. The predicted
prevalence of medical cannabis use in epileptic patients ranges
from about 4 % (77 =total patient population in US medical
cannabis program) to about 20 % (310= total patients at a tertiary
Cannabinoids and Epilepsy
epilepsy clinic in Germany) [252,253]. One percent of the med-
ical marijuana users in California (~2500= total patient popula-
tion) use cannabis to control seizures [254]. In a telephone survey
of 136 patients with epilepsy, 21.0 % were active users, 13.0 %
were frequent users, 8.1 % were heavy users, and 3.0 % met
Diagnostic and Statistical Manual of Mental Disorders-IV criteria
for marijuana dependence.
The majority of patient and caregiver surveys found either
beneficial effects or no significant effect of cannabis in patients
with epilepsy. In a small, 1976 survey of 300 patients with epi-
lepsy, cannabis usage had no effect on seizure frequency in 30 %
of patients, increased seizures in 1 patient, and decreased seizures
in another [255]. A 1989 12-year retrospective study reported
10 % of 47 patients with recreational drug-induced [tonicclon-
ic] seizureshad consumed cannabis prior to seizures, although
this was confounded by recent cocaine, amphetamine, or LSD
(lysergic acid diethylamide) usage. No seizures were reported
following cannabis use alone [256]. A single epidemiological
study provided limited evidence that cannabis may possess anti-
seizure properties in humans. In a study of i llicit drug use
and new-onset seizures in Harlem utilizing a casecontrol
methodology, cannabis used within 90 days before hospi-
talization was associated with a 2.8-fold decreased risk of
first seizures among men but not women [257]. In a tele-
phone survey of adult patients from a tertiary care epilep-
sy center, most active users reported beneficial effects on
seizures (68 % reduced severity, 54 % reduced incidence),
and24%ofallsubjectsbelievedmarijuanawasaneffec-
tive therapy for epilepsy. No patient reported a worsening
of seizures with cannabis use [258]. The majority (84 %)
of patients in a German tertiary care center reported that
cannabis had no effect on their seizure control [253].
0
25
50
75
100
125
150
%
No Significant Effe ct
Mixed Effect
Pro-Convulsive Effect
Anti-Convulsive Effect
Modulators
of the Endo-
Canna binoid
System
CB1R
Agonis ts
CB1R
Antagonists
9
-THC CBD/
CBDV
Cannabinoids and Preclinical Seizure Models
a
b
Modulators of
the Endo-
cannabinoid
System
CB1R
Agonists
CB1R
Antagonists
9-THC CBD/
CBDV
# of Species 32362
# of Discrete
Conditions/
Models
13 69 18 34 41
Anti-
convulsant
6
(46.2%)
47
(68.1%)
1
(5.6%)
21
(61.8%)
33
(80.5%)
Pro-
convulsant
0
(0%)
2
(2.9%)
7
(38.9%)
1
(2.9%)
0
(0%)
Mixed Effect 3
(23.1%)
5
(7.2%)
0
(0%)
1
(2.9%)
0
(0%)
No
Significant
Effect
4
(30.8%)
15
(21.7%)
10
(55.6%)
11
(32.4%)
8
(19.5%)
Diazepam
Valproate
Carbamazepine
Oxcarbazepine
Phenobarbital
Lamotrigine
Pregabalin
Topiramate
Clonazepam
Ethosuxamide
Clobazam
Gabapentin
Lacosamide
Tiagabine
Memantine
Phenytoin
Levetiracetam
Modulators of the
eCB System
CB1R Agonists
CB1R Antagonists
9-THC
CBD/CBDV
URB597
AM404
WIN 55, 212
ACEA
SR141716A
AM251
Legend
Increased effect
Reduced effect
No significant effect
Mixed effect
Not tested
299
299
299 174, -7 172, -5 175 171, -2 177 171, -5 171 174 171 174, -7 177 174 172 177 177
173 170, -3 169, 173 170 170 170 170 170 173 319
299 299
*
326
179
292 181181
179, 181
Fig. 4 Summary of cannabinoids and preclinical seizure models. (A)
Composite data from 175 preclinical seizure models (e.g., maximal
electroshock, kainic acid) or discrete experimental designs (e.g., with
combined antiseizure medications). Pro-/antiseizure effects are
subclassified by given intervention: 1) modulators of the
endocannabinoid (eCB) system (e.g., fatty acid amide hydrolase
inhibitor URB597); 2) cannabinoid type 1 receptor (CB
1
R) agonists
(e.g., WIN55212-2); 3) CB
1
R antagonists (e.g., SR141716A); 4) Δ
9
-
tetrahydrocannabinol (Δ
9
-THC); and 5) cannabidiol (CBD)/
cannabidivarin (CBDV). (See Supplementary Material for complete
description of preclinical studies.) (B) Summary of preclinical data on
cannabinoid interactions with antiseizure medications. Sources
indicated in boxes. *Recent evidence from a phase I clinical trial
suggests that CBD/CBDV elevates serum concentrations of clobazam
and N-desmethylclobazam in human pediatric patients with treatment-
resistant epilepsy [292]. ACEA = arachidonyl-2'-chloroethylamide
A 2013 survey of 19 parents of children with treatment-
resistant epilepsy investigated the use of high CBD:Δ9-THC
ratio artisanal marijuana products. These parents were primarily
identified from social media and included 12 children with
Dravet syndrome (DS). Of the 12 children with DS, parents
reported that 5 (42 %) experienced a> 80 % reduction in seizure
frequency and 2 (11 %) reported complete seizure freedom. The
single child with LennoxGastaut syndrome (LGS) was reported
to have a>80 % reduction in seizure frequency. In addition to
seizure control, parents reported positive effects of increased
alertness (74 %), better mood (79 %), improved sleep (68 %),
and decreased self-stimulation (32 %), and rare AEs of drowsi-
ness (37 %) and fatigue (16 %) [259]. A more recent retrospec-
tive case study described 75 patients from Colorado with
treatment-resistant epilepsy who moved to Colorado for oral
cannabis extract treatment. Oral cannabis extract treatment con-
trolled seizures in 57 % of patients, reduced seizures by >50 % in
33 % of patients, and showed greater effectiveness in patients
with LGS (88.9 %) than in patients with DS (23.0 %). Reported
additional benefits included improved behavior/alertness (33 %),
language (10 %), and motor skills (10 %), as well as rare AEs of
increased seizures (13 %) and somnolence/fatigue (12 %). Inter-
estingly, the study also reported a significant, independent pla-
cebo effectof families moving to Colorado for treatment (see
Placebo Effect)[260]. Collectively, these surveys suggest a
predominantly antiseizure (or no significant) effect of cannabis
usage. However, it is essential to consider the limitations of sub-
jective self-reporting, potentially biased sampling of patient ad-
vocacy groups (over-reporting positive effects), and uncontrolled
differences in CBD:Δ9-THC content in various strains of can-
nabis in these studies.
Clinical Trials
A recent Cochrane review assessed 4 primary clinical trials to
examine the efficacy of medical marijuana in seizure control
(summarized in Table 2, adapted from [11], [13]). Two of
these studies demonstrated a partial antiseizure effect of
CBD [261,262], while 2 showed no significant effect [263,
264]. However, all 4 studies included significant limitations,
including low study sizes, insufficient blinding or randomiza-
tion, or incomplete data sets. The authors of the Cochrane
review and a recent meta-analysis from the American Acade-
my of Neurology both emphasized the need for follow-up
placebo-controlled, blinded, randomized clinical trials exam-
ining the role of CBD in seizure control [12,13].
Phase I Clinical Trial for CBD in Treatment-resistant
Epilepsy
Preliminary preclinical and clinical evidence reveal the thera-
peutic potential of CBD to reduce seizures with high
tolerability and low toxicity. Accordingly, CBD represents a
highly desirable treatment alternative for patients with early-
onset, severe epilepsy such as DS and LGS. In addition to
pharmacoresistant seizures, these patients suffer from severe
neurodevelopmental delay, intellectual disability, autism, mo-
tor impairments, and significant morbidity and mortality [265,
266]. As patients with DS and LGS require effective and
better-tolerated therapies and represent relatively homoge-
neous populations, they stand out as candidates for an initial
trial of CBD safety and efficacy.
Study Design and Results
Investigator-initiated open-label studies at 10 epilepsy centers
using Epidiolex (GWPharma, Salisbury, UK; 99 % CBD)
collected data on 213 patients with treatment-resistant epilep-
sies. This predominantly pediatric population had a mean age
of 10.8 years (range 2.026.0 years). CBD was added to
existing AEDs; there was an average of 3 concomitant AEDs.
The average baseline was 60 per month for total seizures and
30 per month for convulsive seizures.
The primary goal of the study was to assess safety but
seizure diaries were obtained for convulsive, drop, and total
seizures to provide a potential signal regarding efficacy.
Twelve-week or longer continuous exposure data were obtain-
ed for 137 patients and were used in efficacy measures. The
most common epilepsy etiologies were DS and LGS syn-
dromes; others included Aicardi syndrome, Doose syndrome,
tuberous sclerosis complex, CDKL5, Dup15q syndrome, and
many others. At week 12, total convulsive and nonconvulsive
seizures showed a median percent reduction from baseline of
54 %, and total convulsive seizures showed a median percent
reduction from baseline of 51 %. In patients with DS (n=23),
CBD reduced convulsive seizure frequency by 53 %, and
16 % of DS reached complete convulsive seizure freedom
by week 12. Atonic seizure frequency among patients with
LGS (n=10) was reduced by a median of 52 % at week 12.
AEs> 10 % included somnolence (21 %), diarrhea (17 %),
fatigue (17 %), and decreased appetite (16 %). Nine patients
(4 %) were discontinued for AEs. The investigators concluded
that CBD reduced seizure frequency across multiple drug-
resistant epilepsy syndromes and seizure types and was gen-
erally well-tolerated in the open-label study. Randomized con-
trolled trials (RCTs) are now ongoing for DS and LGS.
Safety Issues
There is a strong tendency to equate cannabis as a natural
therapywith cannabis as a safe therapy. This a priori as-
sumptionthe naturalistic fallacyis countered by many in-
stances of toxic or deadly plants (e.g., amotoxins in mush-
rooms) and animals (e.g., tetrodotoxin in puffer fish). A more
muted naturalistic view is that if side effects occur with
Cannabinoids and Epilepsy
cannabis, they would be less severe than those from drugs
produced by the pharmaceutical industry. A recent Epilepsia
survey of 776 individuals found that 98 % of the general
public supported the use of medical marijuana for severe cases
of epilepsy, compared with only 48 % of epileptologists. Sim-
ilarly, the majority of the public and a minority of
epileptologists thought that there was sufficient safety (96 %
vs 34 %) and efficacy (95 % vs 28 %) data for medical mar-
ijuana use in severe epilepsy. This significant disparity in
opinion between professionals and the lay public, possibly
swayed by the appeal of natural remedies, emphasizes an in-
creased need for further research and public education regard-
ing medicinal cannabis and epilepsy [267].
As with efficacy, the most valid assessment of side effects is
with RCTs. RCT data on the safety of Δ9-THC and CBD in
adults comes from trials of cannabinoid-containing medications,
including nabixomols [Sativex (GWPharma) 1:1 Δ9-
THC:CBD], purified cannabis extracts [Cannador, Institute for
Clinical Research, IKF, Berlin, Germany, (2:1 Δ9-THC:CBD)],
synthetic Δ9-THC analogues Dronabinol and Nabilone. These
drugs have been approved by many international regulatory
agencies. In a meta-analysis of 1619 patients treated with
nabiximols for neurological indications (mainly pain, spasticity,
spasm, or tremor) for 6 months or less, 6.9 % of those on can-
nabinoid therapies were discontinued because of adverse effects
versus 2.25 % in the placebo groups [12]. Adverse effects occur-
ring in at least 2 studies included nausea, dizziness, increased
weakness, behavioral or mood changes, hallucinations, suicidal
ideation, fatigue, and feeling of intoxication. No deaths from
overdose were reported [12]. However, our knowledge on the
safety of these compounds in children is very limited.
The adverse health effects of recreational cannabis use were
recently reviewed [268]. Δ9-THC is presumed to be the major
cannabinoid resulting in adverse acute and chronic health effects
of cannabis. The 4-fold increase in Δ9-THC content of confis-
cated cannabis in the last 20 years is associated with increased
acute complications. In 2011, there were 129,000 emergency
department visits for cannabis alone and 327,000 additional visits
for cannabis in combination with other drugs. From 2004 to
2011, the rate of emergency department visits for cannabis tox-
icity doubled [268]. Short-term use can impair short-term mem-
ory, coordination, and judgment. In high doses, paranoia and
psychosis can occur [137,269]. Long-term use of recreational
cannabis in adolescents is associated with addiction (9 % overall
but 17 % among adolescents) and impaired cognitive and aca-
demic performance [270274]. Additionally, cannabis treatment
in animal and human studies altered brain development (espe-
cially with use in early childhood) and structure [272,275277],
creating long-lasting functional and structural brain abnormalities
[277279]. Early and/or heavy cannabis use is associated with
neurochemical abnormalities on magnetic resonance spectrosco-
py [272], impaired maintenance of neuronal cytoskeleton dy-
namics [277], decreased white matter development or integrity
[272,275,276], increased impulsivity [276], and abnormal acti-
vation patterns during cognitive tasks on functional magnetic
resonance imaging [272,280]. In patients with multiple sclerosis,
use of cannabis is associated with impaired cognition and acti-
vation patterns on functional magnetic resonance imaging [281].
Further research is required to determine the short- and long-term
effects of CBD alone, which may have lower toxicity than whole
plant cannabis or Δ9-THC.
Cannabidiol Formulations, Pharmacokinetics,
Pharmacodynamics, and DrugDrug Interactions
We are aware of 3 pharmaceutical products that are currently
in trials or in development: 1) Epidiolex (99 % CBD derived
from C. sativa plants, in a strawberry-flavored sesame oil), 2)
synthetic CBD from Insys Therapeutics (Chandler, AZ,
USA), and 3) Transdermal CBD gel from Zynerba Pharma-
ceuticals (Devon, PA, USA). Other CBD-containing products
are available commercially and obtained online [e.g., Realm
of Caring's Charlotte's Web (whole cannabis extract contain-
ing 50 mg/ml CBD)]. However, the quality control and con-
sistency of these products may vary considerably. Indeed, a
recent study by the US Food and Drug Administration tested
18 products, claimed to contain CBD, made by 6 companies.
Of these, 8 contained no CBD, 9 contained <1 % CBD, and 1
contained 2.6 % CBD (http://www.fda.gov/NewsEvents/
PublicHealthFocus/ucm435591.htm).
Because lipophilic cannabinoids (including CBD) have low
water solubility, CBD is traditionally delivered orally in either
an oil-based capsule or sublingual spray, permitting less vari-
able pharmacokinetics in gastrointestinal absorption. A single
10-mg dose of nabiximols (equal parts CBD and Δ9-THC) in
humans produces a maximum serum concentration (C
max
)of
3.0±3.1 μg/l (buccal) [2.5±1.8 μg/l (sublingual)] and maxi-
mum time (T
max
)of2.8±1.3h(buccal)[1.6±0.7h(sublin-
gual)] [282]. CBD is primarily protein-bound in the blood,
and preferentially deposits inbrainandadiposetissue[283].
The cannabinoids are primarily metabolized by the liver
cytochrome P-450 (CYP-450) enzymes. Both Δ9-THC and
CBD can inhibit CYP-450 metabolic activity, particularly the
CYP2C isozymes at low concentrations and CYP3A4 iso-
zymes at higher concentrations [284289]. CYP2C and
CYP3A4 are induced by carbamazepine, topiramate, and phe-
nytoin, and inhibited valproate and other drugs [290]. The
cannabinoids, particularly CBD, can inhibit other isozymes,
including 2D6 and 1A1 [285,291]. Therefore, use of Δ9-
THC or CBD could potentially contribute to bidirectional
drugdrug interactions with antiepileptic and other drugs. In
our open-label CBD study, patients treated with CBD had
elevated levels of the nordesmethyl metabolite of clobazam
[292], which may account for a portion of the apparent seda-
tion, as well as efficacy, of CBD.
Tab le 2 Clinical trials of cannabidiol (CBD) and epilepsy (adapted from [11,13])
Study Seizure type Population
size
Treat ment
(subjects per group)
Continued
AEDs?
Duration Outcome Toxicity Limitations
Mechoulam and
Carlini [261]
Treatment-resistant,
temporal lobe epilespy
9 CBD, 200 mg/day (4)
Placebo (5)
NS 3 months CBD: seizure free
(2), partial improvement
(1), no change (1);
placebo: no change (4)
None No baseline seizure frequency; no
definition of improvement; unclear
if AEDs were changed; not truly
randomized or blinded; unknown
if groups were matched
Cunha et al. [262] Treatment-resistant,
temporal lobe epilespy
15* CBD 200300 mg/day (8*)
Placebo (8*)
Yes 3 18 weeks CBD: near seizure freedom
(4), partial improvement
(3), no change (1); placebo:
no change (7), partial
improvement (1)
Somnolence Not clearly blinded (1 patient
transferred groups); doses were
adjusted in CBD group, not in
placebo; CBD group received
longer average treatment
Ames and
Cridland [263]
Treatment-resistant
epilepsy, intellectual/
developmental disability
12 CBD 300 mg/day
for 1 week; 200 mg/day
for 3 weeks (6?)
Placebo (6?)
NS 4 weeks No difference between CBD
and placebo
Somnolence Brief letter to the editor, details
lacking on specifics; discontinued
owing to technical difficulties
in preparing the drug
Trembly and
Sherman [264]
Treatment-resistant epilepsy 1012
CBD 100 mg once
daily
Placebo
Yes 3 months baseline, 6 months
CBD or placebo, then
6 months crossover to
alternative treatment
No difference between CBD
and placebo (seizure frequency
or cognitive/behavioral tests)
None Differences in sample size reporting;
dada reported are incomplete
(conference abstract)
AEDs = antiepileptic drugs; NS = not stated
*1 patient switched groups after 1 month
Abstract and subsequent book chapters have different numbers
Only truly double-blind study
Cannabinoids and Epilepsy
Placebo Effect
The magnitude of the placebo response is related to the power
of belief. Given the social and mainstream media attention se-
lectively reporting dramatic benefits of artisanal cannabis prep-
arations for children with epilepsy, there are high expectations
on the part of many parents. The potent role of the placebo
response was suggested by a recent survey of parents whose
children with epilepsy who were cared for at Colorado Chil-
drens Hospital. A beneficial response (>50 % seizure reduc-
tion) was reported 3 times more often by parents who moved to
the state compared with those who were long-time residents
[260]. No differences in epilepsy syndrome, type of artisanal
preparation, or other factor could account for this difference.
While studies have reported a significant placebo response
in adult patients (such as those with ParkinsonsDisease
[293]), placebo response rates are particularly high among
children and adolescents in a subset of disorders, including
psychiatric (anxiety, major depression, and obsessive compul-
sive and attention deficit disorders), medical (asthma), and
painful (migraine, gastrointestinal) conditions [294,295]. As
the current RCTs of CBD primarily target children with severe
epilepsy, this may be an important issue. Among patients with
treatment-resistant focal epilepsy, a meta-analysis found that
the placebo response in children (19.9 %) was significantly
higher than in adults (9.9 %), while the response to the AED
was not statistically different in children (37.2 %) and adults
(30.4 %) [296]. In one predominantly pediatric LGS trial,
seizures were reduced in 63 % of placebo-treated patients
and 75 % of drug-treated patients [297]. Paradoxically, the
intense interest and strong beliefs in the efficacy of cannabis
for epilepsy may elevate placebo responses and make it more
difficult to demonstrate a true benefit in RCTs.
Legal/Ethical Concerns
The Drug Enforcement Agency (DEA) classifies cannabis and
products derived from cannabis plants as Schedule I drugs.
Schedule I drugs have a high potential for abuse and no cur-
rently accepted medical use; they are the most dangerous
drugs of all the drug schedules with potentially severe psycho-
logical or physical dependence (DEA website; http://www.
dea.gov/druginfo/ds.shtml). It is thus paradoxical that opiates
and benzodiazepines, which have a much greater potential for
psychological and physical dependence than cannabis, are
classified as Schedule II drugs. With regard to the DEAs
claimthat cannabis-derived drugs have no currently accept-
ed medical use, therapies such as nabiximols (CBD and Δ9-
THC) and other products have been approved by regulatory
agencies in >20 countries. These approvals are based on RCTs
that establish efficacy and a favorable safety profile, including
a low potential for abuse [228,298].
The Schedule I categorization makes it challenging for inves-
tigators to study cannabis-derived cannabinoids in basic and clin-
ical science. There is often a long and costly process to secure
approvals and inspections to obtain cannabinoids, purchase a
large safe, the weight of which may require clearance from en-
gineers, and add security systems to the room and building in
which they are stored. The Schedule I designation often prevents
patients who live in developmental centers or residential homes
from participating in clinical trials. The threshold of effort for
basic and clinical investigators to study cannabinoids remains
as high as ever, while the availability of these substances for
parents to give is expanding rapidly. This has created a widening
gap between knowledge and exposure, an especially relevant
concern in children for whom safety data are largely lacking.
Conclusion
For over a millennium, pre-clinical and clinical evidence have
shown that cannabinoids such as CBD can be used to reduce
seizures effectively, particularly in patients with treatment-
resistant epilepsy. However, many questions still remain (see
Box 1) regarding the mechanism, safety, and efficacy of canna-
binoids in short- and long-term use. Future basic science re-
search and planned multicenter, placebo-controlled clinical tri-
als will provide insight into cannabinoid function and the po-
tential neuroprotective effects of the endocannabinoid system.
These findings will increase our mechanistic understanding of
seizures and may provide novel, targeted therapeutics for
epilepsy.
Box 1 Unanswered questions and directions for future studies
1. How do the pro and anti-epileptic effects of cannabis change with
development? Are there age-specific differences in responsiveness,
side effects, and target receptor expression?
2. What are the long-term effects of cannabis/cannabidiol use?
3. Are certain types of seizures or genetic channelopathies more likely to
respond to cannabidiol than others?
4. What is the safety of cannabidiol in patients with special conditions
(pregnancy, recent or planned surgery, vagus nerve stimulation, etc.)?
5. How do synthetic cannabinoids (spiceor K2)dysregulatethe
central nervous system to induce seizures? What is their relative safety
andtoxicityrelativetocannabis?
Acknowledgments Drs. Orrin Devinsky and Ben Whalley have re-
ceived research support from GW Pharmaceuticals. We acknowledge
FACES (Finding a Cure for Epilepsy and Seizures) for generous help in
the preparation of this manuscript. R.W.T. is supported by grants from the
National Institute of Mental Health (5R37MH071739) and the National
Institute of Neurological Disorders and Stroke (5R01NS074785,
5R01NS024067).
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... Cannabidiol (CBD) is a non-psychoactive cannabinoid known to have therapeutic effects in several neurological disorders, including epileptic syndromes [8][9][10][11][12]. In animal models, CBD has been shown to have anticonvulsant effects on spontaneous recurrent seizures and to attenuate the behavioral manifestations in several seizure models [13][14][15][16][17]. Cannabidiol reduced pentylenetetrazole (PTZ)-induced seizures in adult male mice [17], and had protective effects against seizures and neuronal death following pilocarpineinduced status epilepticus in adult male rats [13] and intrahippocampal kainic acid-induced seizures in juvenile male rats [14]. ...
... When seizures are induced by chemoconvulsants or electrical stimulation, pro-inflammatory cytokines are elevated, along with other inflammatory mediators, in brain areas associated with epileptic activity [21,22]. The beneficial effects of CBD appear to be related to neuroprotective mechanisms that inhibit excessive inflammatory responses and reduce proinflammatory cytokines, such as IL1-b, which in turn reduces seizure severity [12,[23][24][25][26]. ...