Risk assessment of ritual use of oral
dimethyltryptamine (DMT) and harmala alkaloids
Robert S. Gable
Claremont Graduate University, Claremont, CA, USA
Aim To extend previous reviews by assessing the acute systemic toxicity and psychological hazards of a dimethyl-
tryptamine and b-carboline brew (ayahuasca/hoasca) used in religious ceremonies. Method A systematic literature
search, supplemented by interviews with ceremony participants. Results No laboratory animal models were located
that tested the acute toxicity or the abuse potential of ayahuasca. Separate animal studies of the median lethal dose of
dimethyltryptamine (DMT) and of several harmala alkaloids indicated that a lethal dose of these substances in humans
is probably greater than 20 times the typical ceremonial dose. Adverse health effects may occur from casual use of
ayahuasca, particularly when serotonergic substances are used in conjunction. DMT is capable of inducing aversive
psychological reactions or transient psychotic episodes that resolve spontaneously in a few hours. There was no
evidence that ayahuasca has substantial or persistent abuse potential. Long-term psychological beneﬁts have been
documented when ayahuasca is used in a well-established social context. Conclusion A decoction of DMT and
harmala alkaloids used in religious ceremonies has a safety margin comparable to codeine, mescaline or methadone.
The dependence potential of oral DMT and the risk of sustained psychological disturbance are minimal.
Keywords Abuse potential, ayahuasca, dimethyltryptamine, DMT, toxicity.
Correspondence to: Robert S. Gable, 2738 Fulton Street, Berkeley, CA 94705, USA. E-mail: email@example.com
Submitted 19 February 2006; initial review completed 12 June 2006; ﬁnal version accepted 11 July 2006
Among substances suspected of having abuse potential,
N,N-dimethyltryptamine (DMT) has received relatively
little attention. However, in 2006, a case was decided by
the US Supreme Court that involved the question as to
whether a DMT and b-carboline plant decoction
(ayahuasca/hoasca) is safe for ceremonial use by
members of a small spiritualist Christian church . The
church prevailed in a unanimous decision in part
because the government, which opposed the use of DMT,
could not meet its burden of showing that hoasca posed a
serious health risk to church members. This paper is a
review of the known acute systemic risks of oral DMT
and concomitantly ingested harmala alkaloids.
Risk refers to the quantiﬁed probability of a future
harm. The concept of risk requires a causal relationship
between a hazard (such as a drug overdose) and a known
unfavorable outcome (such as illness or death). In order
to establish this causal relationship, a stimulus must
produce an effect more often than would normally occur
in the absence of that stimulus. Thus, the probabilistic
estimate of risk is often an intrinsic aspect of the research
process by which a causal relation is demonstrated. In the
case of oral DMT, probabilistic estimates can be made for
very few of the drug’s effects at the present time. Some
effects, both favorable and unfavorable, that have been
attributed to DMT may amount to little more than plau-
sible associations. Given the limited amount of published
scientiﬁc data regarding oral DMT, this paper takes a
broad view of potential hazards with the expectation that
future research will better establish the risks that might
Ayahuasca (EYE-ah-WAS-ka) or hoasca (WAS-ka) is a
mixture consisting essentially of two compounds. One is
an amine—the simplest and most common being the
tryptamine DMT. The second compound is a monoamine
oxidase inhibitor (MAOI) in the form of a b-carboline
such as harmine, harmaline or tetrahydroharmine. DMT
alone has predicable and signiﬁcant psychological
© 2007 The Author. Journal compilation © 2007 Society for the Study of Addiction Addiction,102, 24–34
activity when smoked, injected or insufﬂated; but when
used orally it only becomes, or remains, psychoactive in
combination with an MAOI [2–4].
Ayahuasca/hoasca is prepared customarily by com-
bining the leaves from the Psychotria viridis bush along
with bark scraped from the stem of the Banisteriopsis caapi
vine [4,5]. The ingredients are boiled for several hours
and then decanted. The resulting thick, brown, oily liquid
(referred to colloquially in some locales as ‘vine of the
souls’ or ‘vine of the dead’) has been used throughout the
Amazon Basin as a medicinal and ceremonial beverage
since antiquity .
Various sacramental brews containing DMT and
b-carbolines, used primarily in Amazonia, are generally
known as ayahuasca. The term ‘hoasca’, a Portuguese
transliteration of ayahuasca, is limited historically and
properly to a subclass of ayahuasca beverages associated
with a speciﬁc sacramental use (, chapter 16, ‘Hoasca
versus Ayahuasca’). The most commonly recognized sac-
ramental use of ayahuasca occurs among members of
two churches in Brazil: the O Centro Espirita Beneﬁciente
Uniao Do Vegetal (UDV), founded in 1961 with approxi-
mately 8000 current members, and the Santo Daime,
founded around 1940 with approximately 2500 current
members. Hoasca preparation occurs as part of a reli-
gious ritual known as preparo in the UDV. The Santo
Daime church also uses a decoction of P. viridis and B.
cappi called santo daime or simply daime, and its prepara-
tion ritual is known as feitio . In the present paper,
‘hoasca’ will refer exclusively to UDV’s sacramental
decoction of plant DMT and harmala alkaloids.
The UDV cultivates the plants that are used in their
hoasca ‘tea’ on land that the church owns and maintains
for that purpose. The preparo ceremony can involve up to
200 people working together at a UDV temple over a
period of several days. Photographs of the process appear
in Metzner (, pp. 20, 21, 205). In 1992, the Brazilian
Federal Narcotics Council granted legal status for the use
of hoasca in religious contexts.
The present paper extends previous reviews [9,10] that
compared the acute lethal toxicity and the abuse poten-
tial of various psychoactive substances. A simple induc-
tive and iterative procedure was used to locate English-
language serial publications, from 1969 to 2005,
accessible through six databases: Biosis Previews, Digital
Dissertations, Google, PsychAbstracts, Pubmed and
Toxline. The Science Citation Index was occasionally used
to follow-up unusually salient reports. Six primary search
terms—‘ayahuasca’, ‘hoasca’, ‘dimethyltryptamine’,
‘harmine’, ‘harmaline’ and ‘tetrahydroharmine’—were
keyed into the search engines and scanned for the topics
‘overdose’, ‘lethal dose’, ‘lethality’, ‘toxicity’, ‘death’,
‘therapeutic index’, ‘abuse potential’ and ‘dependency’.
The most relevant books, reference works and special
journal issues (e.g. [5,8,11–12]) were also consulted,
thus expanding the information sources beyond those
located only by the search descriptors. In 2003, this
author met with 16 people 2 hours before and periodi-
cally for 12 hours after an ayahuasca ceremony led by a
Peruvian shaman. In 2005, the author had access to a
detailed legal document  consisting of more than
1000 pages that served as the evidentiary basis for health
information about hoasca for use in the US Supreme
Court case mentioned previously .
An item retrieved during the literature search was
considered potentially relevant if it met at least two of
four criteria: (1) DMT or one of the monoamine oxidase
inhibitors was quantiﬁed with respect to an effective or a
toxic dose; (2) the health status of the individual was
indicated; (3) the possible use of concomitant substances
was mentioned; and (4) the data source appeared to be
technical or scholarly in nature.
Fewer than 100 scholarly articles that focused speciﬁcally
on ayahuasca/hoasca were located in English-language
serial publications. The majority of these ar ticles involved
inapplicable topics such as neurological experiments, eth-
nographic descriptions or potential medical uses. The
descriptors ‘dimethyltryptamine’, ‘harmine’, ‘harmaline’
and ‘tetrahydroharmine’ resulted in approximately 200
citations retrieved from the databases, excluding Google
and the Science Citation Index. However, after cross-
indexing the keyword ‘toxicity’ with these four descrip-
tors, the number of non-redundant citations that met the
screening criteria was fewer than 25. When all relevant
citations from serial publications were supplemented
with excerpts from scholarly books, monographs and
published reports, the total number of documents printed
and ﬁled was approximately 140.
Description of ayahuasca/hoasca brew
The DMT alkaloid has been reported as ranging from
0.1% to 0.66% dry weight in P. viridis leaves . Sur-
prisingly, the DMT in leaf samples from a single P. viridis
plant has been shown to vary from approximately 3 mg/g
to 9.5 mg/g dry weight in the course of one day. The
concentrations of the b-carboline alkaloids in B. caapi
have been reported as ranging from 0.05% to 1.95% dry
Substantial variation in concentrations and propor-
tions of the constituents of ayahuasca brews can be
expected as a consequence of the varying chemical
Risk assessment of oral DMT 25
© 2007 The Author. Journal compilation © 2007 Society for the Study of Addiction Addiction,102, 24–34
composition of the source plants as well as different
methods of preparation. Average DMT doses in assayed
ayahuasca brews have ranged from 8.8 mg  to 42 mg
. The content of other alkaloids also varied widely
(see Table 1). This ﬁvefold variance of DMT quantities in
ayahuasca brews might be compared to the common
ﬁvefold variance in caffeinated beverages which range
typically from 0.2 mg/ml of caffeine in green tea to
1.0 mg/ml of caffeine in ﬁltered coffee .
The ritual structure in which hoasca is ingested
provides some control of dosage and subsequent psycho-
logical effects. Because the natural sources used in prepa-
ration of the tea does not allow UDV members to
standardize their hoasca brew with respect to DMT or
b-carbolines, the person conducting the ceremony drinks
the brew before administering it to UDV members as a
means of testing for potency. Different amounts of the
brew are initially offered to individual participants and,
depending upon reactions, a participant may be offered a
second cup at his or her request.
A clear distinction must be made between prepara-
tions consisting exclusively of DMT that are injected,
smoked or insufﬂated versus DMT preparations that are
oral mixtures of DMT and MAOI. Injected, smoked or
insufﬂated DMT is noted for its very rapid activity. Peak
cognitive effects last only 3–10 minutes, and normal con-
sciousness returns within 30 minutes. In contrast, initial
somatic effects (e.g. nausea, tingling, increased body tem-
perature) after ingesting oral DMT appear in approxi-
mately 20 minutes, followed by the onset of cognitive
effects that peak between 60 and 120 minutes . The
cognitive effects diminish gradually to a normal state in
approximately 4 hours. At normally used doses, the psy-
chological effects of oral DMT are less intense than those
of injected, smoked or insufﬂated DMT.
A traditional and standard criterion for assessing the
relative toxicity of various substances has been the lethal
dose of a substance. The median lethal dose (LD50)isthe
statistically derived quantity of a substance given in a
single dose that causes death in 50% of the experimental
animals. One early study by Trevan  used more than
900 mice to assess the toxicity of cocaine and of insulin.
More humane standards, such as the minimal lethal dose
or the maximal non-lethal dose, are being gradually
implemented by toxicologists, although many regulatory
agencies still rely on the traditional LD50. Some test pro-
cedures have been advocated that can reduce the number
of killed animals to as few as six .
The criterion of single-dose acute lethality is an
extremely limited estimate of human toxicity because it
does not take into account variables such as interspecies
differences, repeated dosing, environmental conditions,
prior health status and psychological factors. None the
less, the inﬂuence that such variables might have on
an established dose–response relationship should not
obscure the fact that the LD50 is a clearly deﬁned, repli-
cable and important benchmark of toxicity.
The LD50 of DMT in mice is reported as 47 mg/kg
intraperitoneally and 32 mg/kg intravenously . No
other LD50 data on DMT are believed to be available in
the English language at this time. Five compounds
with structural resemblance to DMT—serotonin,
psilocin, psilocybin, bufotenin and 5-methoxy-N,N-
dimethyltryptamine (5-MeO-DMT)—all have an intra-
venous LD50 among rodents that are similar to, or
substantially less toxic than, DMT. In two rare studies
reporting oral lethal doses of these related compounds,
the LD50 of serotonin in mice was 60 mg/kg ; the
LD50 of 5-MeO-DMT in mice was 278 mg/kg .
The principal b-carboline alkaloids added to DMT in
ayahuasca mixtures are harmine, harmaline and tet-
rahydroharmine. In addition to the B. caapi vine, these
alkaloids can be found in the Peganum harmala (Syrian
rue) shrub, a desert plant native to Central Asia. An
aqueous extract of P. harmala seeds administered orally to
rats resulted in a LD50 of 2 g/kg . Because the
b-carboline admixtures in ayahuasca appear to be less
Table 1 Quantity (in milligrams) of alkaloids in ayahuasca/hoasca brews.1
Dose size 2DMT Harmine Harmaline THH Reference
140 ml 35.6 238 28 150 [2,14,18]
237 ml 33 17 26 Not stated 
60 ml 36 280 25 96 
Gelatin capsule 30 120 None None 
100 ml 9.1 47.5 Trace 4.2 
100 ml38.8 9.2 Not stated 26.5 
Gelatin capsule 42 71 4.8 57 
200 ml 25 30 trace 10 
1Adapted, in part, from Riba . 2Estimated average given to a 70 kg adult in the study referenced. 3Two samples of ayahuasca reported in the same
26 Robert S. Gable
© 2007 The Author. Journal compilation © 2007 Society for the Study of Addiction Addiction,102, 24–34
toxic than DMT, our attention should focus on the DMT
A simple extrapolation of DMT lethality data from
mice to humans is obviously untenable. We are therefore
forced to make some intrepid, but not original, assump-
tions. One traditional rule for scaling unknown differ-
ences between humans and non-human animal species is
simply to assume that humans are 10 times more sensi-
tive, based on body weight, than rodents . To be even
more cautious in light of the absence of LD50 data regard-
ing oral DMT, let us assume that humans are 20 times
more sensitive than rodents. This would result in a
human LD50 of 1.6 mg/kg for DMT administered intrave-
nously, or a total intravenous dose of 112 mg for a typical
70 kg person. Because we are interested in the potential
toxicity of oral DMT in ayahuasca, it is necessary to
convert our estimated intravenous LD50 dose of
1.6 mg/kg DMT to an oral equivalent. Usually, an intra-
venously administered dose of a substance is assumed to
have bioavailability of 100%. The bioavailability of an
oral dose is signiﬁcantly less. An oral DMT dose of
1.0 mg/kg has been reported to increase blood pressure
and heart rate comparable to a 0.1–0.2 mg/kg intrave-
nous dose . With respect to psychological reactions,
an intravenous DMT bolus of 0.4 mg/kg has been docu-
mented carefully as ‘highly psychedelic’ (, p. 138),
while an oral dose of DMT ‘in excess of 2.0 mg/kg’ in
ayahuasca is known to produce maximal effects (,
p. 175). Let us assume an intravenous-to-oral conversion
factor of 1 : 5, based on the assumption that 0.4 mg
intravenously is equivalent to approximately 2.0 mg of
oral DMT. If 1.6 mg/kg is a median lethal dose of
intravenous DMT, then a median lethal dose of oral
DMT would be 8 mg/kg, or a total dose of 560 mg for a
70 kg individual. Note, however, that 560 mg is the esti-
mated median dose, and therefore one-half of potential
DMT fatalities would occur at an oral dose less than
A safety ratio or safety margin for ayahuasca can be
estimated by comparing the calculated LD50 to the cus-
tomary effective dose. The average ceremonial dose of
DMT, as listed in Table 1, is about 27 mg; therefore, the
safety margin for ayahuasca is approximately 20 (560/
Intravenous DMT is known to increase heart rate
rapidly, as well as systolic and diastolic blood pressure.
Strassman & Qualls  reported that a 0.4 mg/kg dose
of DMT at the postinjection interval of 2 minutes raised
the heart rate approximately 26 beats per minute (bpm),
and raised systolic pressure by 35 mmHg and diastolic
pressure by 30 mmHg. DMT plus b-carbolines in
ayahuasca ingested in doses ranging from 0.48 mg/kg
to 1.0 mg/kg were shown to induce peak increases in
heart rate and blood pressure at about one-third that of
intravenous DMT after 90 and 120 minutes [3,20].
Table 2 summarizes these data along with comparisons
of some commonly used psychoactive substances. The
reference cited for each of the substances in Table 2 is
the primary, but not exclusive, source of the information
On the basis of acute heart rate and blood pressure
data, the hemodynamic effects of a typical dose of
ayahuasca appear to be less potentially hazardous than
many commonly used psychoactive substances.This ten-
tative conclusion assumes that any substance listed in
Table 2 Heart rate and blood pressure increases after administration of DMT or other psychoactive substances.
Intravenous 0.4 2 minutes 26 35 30 2
Oral 0.48 20 minutes 7.4 9.0 9.3 
Oral 0.5 45 minutes 6.4 8.8 10.4 
Oral 0.75 45 minutes 8.0 13.4 9.8 
Oral 1.0 45 minutes 9.2 13.8 8.6 
Alcohol oral 1157 15 minutes 9 -2 1 
Caffeine oral33.3 15 minutes 4.0 5.0 5.0 
Cocaine insufﬂated41.37 55 minutes 17 14 14 
Marijuana smoked (98.6 ng/ml)515 minutes 11.6 (See footnote 6) 
MDMA oral 1.5 60 min 28 25 7 
1Assumes an individual who has not developed toleranceto the indicated substance. 2The reference listed is the primary, but not the exclusive, source for
the data summarized in the table. Some original data were averaged. 3Caffeine powder mixed with grapefruit juice; caffeine dose was equivalent to two
or three cups of instant coffee. 4Administered as 120 mg of white powder (cocaine and lactose) in four ‘lines’, snorted two lines per nostril. 5Plasma THC
from 10 puffs, at 1-minute intervals, from a marijuana cigarette containing 1.75% THC. 6Typically, a slight increase in blood pressure when in a supine
position, a decrease when in an upright position.
Risk assessment of oral DMT 27
Table 2 is administered once by the route and in the
quantity indicated without any concomitant pharmaco-
logically active substances. Occasional stroke, myocardial
infarction and other adverse cardiovascular events can be
expected to be associated with, even if not directly caused
by, the use of ayahuasca or other drugs or foods that
induce acute hemodynamic changes. Individual differ-
ences in metabolism and health status often result in a
wide range of reactions to psychoactive substances. For
example, the heart rate of some of the participants in the
Strassman & Qualls intravenous DMT study  peaked
at 150 bpm, while the heart rate of other participants
was raised no higher than 95 bpm. Furthermore, the
hemodynamic reaction to ayahuasca may depend on the
relative concentrations of DMT to the b-carbolines. In
decoctions tested by Pomilio, Vitale & Ciprian-Ollivier
, an exceptionally low concentration of DMT com-
pared to b-carbolines signiﬁcantly decreased heart rate.
An experiment with dogs found that harmala alkaloids
decreased heart rate but were inconsistent in their effect
on blood pressure .
Because heart rate increases with ayahuasca are so
minimal, the differences may be the result of unrelated
physical activity of the participants. Such changes might
be put into perspective by a study conducted by Hartley,
Lovallo & Whitselt . A beverage containing a 230 mg
dose of caffeine (equivalent to two or three 150 ml cups of
instant coffee) resulted in only negligible increases in
heart rate (4 bpm) and blood pressure (5 mmHg). Later in
the same study, the researchers asked the 77 study par-
ticipants to deliver a 3-minute speech ‘to a video camera
in front of 2 experimenters wearing white coats’ (,
p. 1023). Under this anxiety-producing condition, the
average increase in heart rate was 30 bpm; the systolic
pressure increased an average of 28 mmHg and the dias-
tolic pressure increased an average of 21 mmHg. These
increases are more than twice those occurring with oral
It is generally held that the b-carbolines in ayahuasca
are reversible and highly selective inhibitors of MAO .
The ayahuasca-induced blockade of MAO presumably
allows a larger quantity of serotonin to accumulate in
nerve terminals. Excessive accumulation can produce a
range of adverse physiological symptoms, a ‘serotonin
syndrome’, that includes tremor, diarrhea, autonomic
instability, hyperthermia, sweating, muscle spasms and
possible death .
A few irreversible or non-selective MAO inhibitors
(e.g. phenelzine, tranylcypromine) are associated
strongly with instances of severe serotonin syndrome,
but a large number of opiates, analgesics, tricyclic anti-
depressants, selective serotonin reuptake inhibitors
(SSRIs) and anti-migraine agents have also been
implicated . Individuals who have recently used
ginseng, St John’s wort, dextromethorphan or 3,4-
methylenedioxymethamphetamine (MDMA: ecstasy)
should be cautious about using ayahuasca. Physical dis-
comfort or chronic pain (e.g. backache) may be exacer-
bated by ayahuasca, so potential users should be warned
of this side effect.
Table 3 lists six reported instances in which there
were severe toxic reactions to a psychoactive tryptamine
and/or a b-carboline. The death of a 25-year-old male
caused by amine intoxication  involved 5-MeO-DMT
in addition to DMT and b-carboline. No laboratory
animal studies were located during the present review
that compared the toxicity of orally active 5-MeO-DMT
with orally active DMT. However, 5-MeO-DMT is
reported to be more potent than DMT when smoked 
or taken orally with harmaline . Experienced
researchers have cautioned that these substances should
not be casually interchanged in ayahuasca-like prepara-
tions. The male decedent ingested an unknown quantity
of ‘herbal tonics’ that presumably contained MAOIs in
addition to the DMT and 5MeO-DMT because he was
found to have a blood concentration of tetrahydro-
harmine three times higher than that found among 14
volunteers in a UDV hoasca study . The autopsy was
performed the day after the body was discovered, so
post-mortem drug redistribution makes it difﬁcult to
determine what the peak cardiac or peripheral blood
concentration of tetrahydroharmine or 5-MeO-DMT
might have actually been.
The brieﬂy described fatality of a 71-year-old diabetic
female, listed in Table 3, repor ted that only tobacco leaves
and B. caapi were the constituents of the so-called
‘ayahuasca’ brew . The abstract did not document
the concentration of any b-carbolines, but did report
measurements (at unstated time intervals) of
1900 ng/ml and 710 ng/ml of blood nicotine. These con-
centrations exceed by at least 20 times the average post-
mortem nicotine concentration (34 ng/ml) reported by
Ekwald & Clemedson . ‘The cause of death [of the
71-year-old] was attributed to acute nicotine intoxica-
tion’ (, p. 287). Administration of the unorthodox
nicotine/b-carboline brew by enema to this older person,
who was a non-smoker, precludes the relevance of this
case to typical ayahuasca/hoasca use.
The quantity of P. harma seeds consumed as a single
intoxicant by the two individuals cited in Table 3 [36,37]
should be distinguished from the lower quantity of B.
caapi or P. harmala that is used customarily in an ayahua-
sca brew. These individuals ingested 15–50 times more
b-carboline alkaloids than the 3 g needed to maintain the
oral activity of DMT in an ayahuasca preparation .
Herraiz  has noted that coffee may act as a MAOI, and
the caffeine used by one of the decedents probably inten-
siﬁed the effect of the harmala seeds.
28 Robert S. Gable
The last item listed in Table 3 involved more than 30
individuals between the ages of 20 and 50 years who
‘were participating in a meditation session named
“releasing autohypnosis of forest medicine men”’ (,
p. 50). The published report does not detail the number of
participants who were hospitalized, their gender or their
health status following the ingestion of 100–200 ml of
herbal tea, although all the patients recovered satisfacto-
rily. Fatalities from atropine or scopolamine are rare, and
from harmine are extremely rare. The reported dose of
atropine (4 mg) and of scopolamine (78 mg) received by
the meditation session participants is eight to 13 times
less than the estimated minimal lethal dose of these sub-
stances [40,41]. The severe reactions of at least a few of
the individuals who required mechanical ventilation
was attributed by the report authors to the combined
synergistic actions of harmine, atropine and scopola-
mine. This incident provides additional evidence of the
potential danger of combining pharmacologically active
substances in non-traditional ways.
To this author’s knowledge, there have been no deaths
caused by hoasca or any other traditional DMT/b-
carboline ayahuasca brews. The probability of a toxic
overdose of ayahuasca is seemingly minimized by seroto-
nin’s stimulation of the vagus nerve which, in turn,
induces emesis near the level of an effective ayahuasca
dose. The risk of overdose appears to be related primarily
to the concurrent or prior use of an additional serotoner-
gic substance. People who have an abnormal metabolism
or a compromised health status are obviously at a greater
risk than the normal population, and might prudently
avoid the use of ayahuasca preparations.
Table 3 Acute toxic reactions to ayahuasca-related compounds.
Age/sex Primary Estimated Effects Toxicological Factors Reference
25 m5-MeO-DMT Unknown quantity
of ‘herbal tonics’
Death caused by
Camping in park,
71 F Ayahuasca
Not reported Death caused by
Nicotine 1900 mg/l
and 710 mg/l
100 ml of brew Gross motor tremors,
treatment after 4
No blood chemistry
Daily use of 20 mg
of oral ﬂuoxetine
150 g of P. harmala
Heart rate 100 bpm,
bp 80/40 mmHg,
in few hours after
Gastric ulcer, mild
anemia but other
27 F P. harmala 50 g of P. harmala
resolved in a few
level not reported
Seeds ingested with
A brew of harmine
27 mg, atropine
atropine 4 mg,
Harmine 179 mg/l,
atropine 27 mg/l
1Coffee brews contain b-carboline alkaloids. 2‘More than 30 people aged 20–50 years’. No information provided regarding sex or health status of
Risk assessment of oral DMT 29
The general sequence of psychological and cognitive
responses to ayahuasca is dose-dependent and predict-
able; however, the reactions of any particular individual
at any one session are not. With a medium dose of DMT,
objects in the environment appear to vibrate and increase
in brightness. Rapidly moving patterns and scenes
emerge that are visible with eyes either open or closed.
This experience is referred to as being a ‘visionary state’
(, p. 109). It should be noted that these altered per-
ceptions and cognitions do not usually cause users to
become unaware of their surroundings or lose the ability
to speak coherently. Many users walk to the restroom as a
result of diarrhea.
Riba and colleagues  described the reactions of six
volunteers after they each received doses of 0.05, 0.75
and 01.0 mg/kg of ayahuasca. Results obtained on
Addiction Research Center Inventory (ARCI) scales
showed that as the doses increased, emotions of happi-
ness, sadness, awe and amazement also increased. At
medium and high doses, the volunteers agreed that the
experience seemed similar to a dream and that the sense
of self and the passing of time were deeply affected. For
example, the awareness between individuals can become
so interlinked that typical self-centered judgements about
the contents of another person’s statement are simply
absent. With respect to time perception, some people
report that during an ayahuasca session they are swept
into a conscious state where the usual orderly pro-
gression of time becomes obviously non-existent, and
they experience ‘eternity’ (, p. 45). This sensation/
perception was often associated with the emotion of well-
being, but it can also be accompanied by feelings of terror.
Virtually all data sources indicate that the DMT or
ayahuasca experience has a substantial degree of unpre-
dictability with respect to both aversive and positive
aspects, depending on variables such as dosage, partici-
pant’s intention and setting. Reactions during ayahuasca
ceremonies have ranged from profound calmness  to
anguished cries for forgiveness .
In interviews with 150 informants, Shannon 
reported that, even among individuals without academic
education or philosophical training, the ayahuasca expe-
rience prompted users to reﬂect seriously on the phenom-
ena of life, nature and human consciousness. A pilot
study by Grob and colleagues  comparing 15
members of the UDV with a control group of 15 non-
members revealed that the UDV members showed more
rigidity, regimentation, reﬂection and word-recall on
various tests than non-members. According to Grob
(, p. 90), ‘all of the 15 UDV subjects claimed that
their experience with ritual use of hoasca as a psychoac-
tive ritual sacrament had had a profound [positive]
impact on the course of their lives’. Most of the UDV
members had a history of moderate to severe alcohol use
prior to joining the UDV. This variable was not reported
for the control group, so the generalization of this study’s
result to the general population is uncertain.
Certain perceptual characteristics of the ayahuasca
experience overlap those of schizophrenia, and a few
researchers have reported that urinary or blood levels of
DMT are above normal in schizophrenic individuals
[48,49]. This has led to the hypothesis that the non-
destruction of dimethylated indolealkyalamines such as
DMT could play a role in progressive deterioration of cog-
nitive processes . Other studies have produced con-
ﬂicting results and alternative conclusions [51,52]. Jacob
& Presti (, p. 935) proposed that increased DMT,
acting at the G-protein-coupled trace amine receptor,
might actually serve in schizophrenics as ‘a homeostatic
response to calm or suppress psychotic activity, rather
than exacerbate it’. At present, the proposition that
endogenous DMT in schizophrenia is related biochemi-
cally to ayahuasca-induced states of consciousness
remains a speculative hypothesis.
The hallucinogenic effect of ayahuasca and other
tryptamine derivatives can precipitate severe adverse psy-
chological reactions, and this is especially true when
administered outside established ceremonial practices
. For example, two cases of unsupervised use of
‘foxy’) have been reported in which an unknown amount
of 5-MeO-DIPT caused sensory hallucinations requiring
several hours of hospitalization [55,56]. Transient psy-
chotic episodes are also known to occur with high doses
of psilocybin  and LSD . LSD, psilocybin and
5-MeO-DIPT are, however, orally active and more potent
than DMT; thus the comparability of these three sub-
stances with DMT is somewhat problematic.
Over a period of 5 years, the medical studies section of
the UDV documented between 13 and 24 cases in which
ayahuasca might have been a contributing factor in a
psychotic incident (, p. 701). The incidents docu-
mented by the UDV occurred from an estimated total of
25 000 servings of the hoasca tea. Although the preva-
lence rate of psychosis or schizophrenia among adults in
the United States varies according to the way in which
diagnostic criteria are applied, the generally accepted
estimate is approximately 1.3% . A reported UDV rate
of psychotic episodes under 1% suggests that the use of
hoasca is not a triggering event for sustained psychosis.
Many or most of the UDV psychotic episodes were tran-
sient in nature and resolved spontaneously (, p. 623).
Dependence and abuse potential
The extent to which ayahuasca might lead to physiologi-
cal dependence or compulsive drug-seeking is an
30 Robert S. Gable
important public health concern. The physiological
dependence potential of DMT and ayahuasca remains to
be documented convincingly. Hallucinogenic substances
structurally similar to DMT are rarely used in a compul-
sive manner that would meet the dependence criteria of
the Diagnostic and Statistical Manual version IV (DSM-IV)
. No studies were located during the present review
that reported that the termination of DMT resulted in an
With respect to drug tolerance, however, a few non-
human animal studies have been conducted that show
varying degrees of tolerance to behavioral and physi-
ological responses over a period of 3 or 4 weeks [61–63].
Little or no tolerance to emotional or autonomic effects
was reported in one study, where human volunteers were
administered DMT four times at 30-minute intervals
, and in another study where DMT was administered
twice daily for 5 days .
Despite the presumed absence of physiological depen-
dency and tolerance, ayahuasca might, nonetheless,
function as a positive reinforcer leading to signiﬁcant
abuse potential. We would want to know, for example,
what proportion of individuals who try DMT or ayahua-
sca once or twice fall into a pattern of chronic use that
they then ﬁnd difﬁcult to quit. The UDV has reported that
15–20% of ﬁrst-time participants in hoasca ceremonies
become UDV members (, p. 700). This subsequent
participation in the UDV church is within the reported
range of ﬁrst-time visitors who become members of
Christian churches in the United States .
Future consumption patterns of ayahuasca are difﬁ-
cult of determine because, in part, the frequency of
present use in the general population is so low, but its
general psychopharmacological proﬁle suggests that it
lacks the abuse potential of amphetamines, cocaine,
opiates or other widely abused substances. Indeed,
accounts of any strong and sustained reinforcing effects
of tryptamine compounds are rare in experimental litera-
ture. More typically, non-human animal studies conclude
that administration of tryptamine derivatives such as
mescaline, psilocybin or DMT result in erratic patterns of
self-administration indicating that ‘these compounds
have weak reinforcing effects, or alternatively, mixed rein-
forcing and aversive effects’ (, p. 156).
Any attempt to characterize the possible acute adverse
health effects of ayahuasca is hampered by the very
limited number of relevant scientiﬁc studies of DMT and
b-carboline decoctions. This situation invites an easy
commingling of facts, speculations, inferences, biases,
conjectures and hidden agendas. Arguments made in the
course of litigation are especially prone to linguistic
obfuscation of empirical weaknesses. None the less, legal
and policy decisions must be made in the light of what
almost always seems to be insufﬁcient evidence. Data
available at this time indicate that the acute systemic tox-
icity of ayahuasca is, by comparison, substantially less
than alcohol. The average acute lethal dose of ethyl
alcohol is well-documented at approximately 330 g ,
10 times the normal recreational dose. The acute lethal
dose of ayahuasca was calculated in this review as 20
times the effective dose. This safety margin is similar to
codeine, mescaline and methadone . Previous esti-
mates of the LD50 of DMT have been as high as 40 or 50
times the customary dose [9,68]; the difference is prima-
rily a result of varying the safety factors that are used to
extrapolate data from non-human laboratory animal
studies to humans.
No acute health hazards, excluding potential seroton-
ergic reactions, have been documented as a routine,
serious threat from ayahuasca when ingested within the
range of customary dosages. Possible chronic health
effects of ayahuasca were not considered in the present
Most public attention is focused on hazards that we
want to avoid, such as accidents, heart attacks or bank-
ruptcy. However, there are hazards that people accept,
perhaps grudgingly, because they perceive a potential
beneﬁt—for instance, crossing a street against the trafﬁc
light, speeding to a hospital with a sick child, agreeing to
an adjustable mortgage. Andritzky  has described the
traditional use of ayahuasca in Amazonia during healing
rituals. A more contemporary North American illustra-
tion was provided by a university administrator who
reported that he previously considered Schedule I drugs
to be a taboo option until he was diagnosed with meta-
static liver cancer . When potential beneﬁts are con-
sidered, the option with the least risk is not necessarily
the best choice.
Indeed, some risks are not merely accepted, but
actively sought. Examples include hang-gliding, road
racing and alcohol drinking contests. Recently, the inter-
net has become a favored source of information for
individuals seeking drug-related experiences. Boyer,
Shannon & Hibberd  have proﬁled 12 adolescents in a
drug treatment program who reported being inﬂuenced
detrimentally by drug information, including a descrip-
tion of Syrian rue, that they found on the internet.
However, the internet also contains descriptions of
intoxicated misadventures that might serve as a caution-
ary tale (e.g. a self-reported ayahuasca overdose caused
by the user’s self-described ‘swaggering arrogance’ (,
p. 1). Virtually all poisoning reports with tryptamine/
MAOI mixtures involve individuals who prepared their
own brew and/or who ingested an additional psychoac-
tive substance [35,39,72,73].
Risk assessment of oral DMT 31
The relative lack of abuse potential of ayahuasca in
social settings seems very plausible. The unpredictable
occurrence of frightening images and thoughts, plus pre-
dictable nausea and diarrhea, makes it a very unlikely
candidate for a ‘club drug’.
Finally, it might be noted that the discipline of toxicol-
ogy has its own queasiness—especially about spiritual
concepts such as ‘transcendence’, ‘ineffability’ and
‘grace’ that often appear in descriptions of ayahuasca
sessions by physically and psychologically healthy indi-
viduals . Many reported experiences are similar to
descriptions of samadhi in advaitan Hinduism, satori in
Zen Buddhism or beatiﬁc vision in Christianity. Such
alleged beneﬁcial experiences lie outside the pathology-
oriented realm of toxicology, but not necessarily or com-
pletely beyond the scientiﬁc requirement of falsiﬁability.
Variations in consciousness are, at least in theory, worthy
of serious scientiﬁc study because of their central place in
This review was funded, in part, by the Life Science
Research Group, Inc., an independent non-proﬁt research
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