Adverse Health Effects of Marijuana Use

Abstract

As marijuana use becomes legal in some states, the dominant public opinion is that marijuana is a harmless source of mood alteration. Although the harms associated with marijuana use have not been well studied, enough information is available to cause concern.

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Dan L. Longo, M.D., Editor
review article
Adverse Health Effects of Marijuana Use
Nora D. Volkow, M.D., Ruben D. Baler, Ph.D., Wilson M. Compton, M.D.,
and Susan R.B. Weiss, Ph.D.
From the National Institute on Drug
Abuse, National Institutes of Health,
Bethesda, MD. Address reprint requests
to Dr. Volkow at the National Institute
on Drug Abuse, 6001 Executive Blvd.,
Rm. 5274, Bethesda, MD 20892, or at
nvolkow@nida.nih.gov.
N Engl J Med 2014;370:2219-27.
DOI: 10.1056/NEJMra1402309
Copyright © 2014 Massachusetts Medical Society.
I
n light of the rapidly shifting landscape regarding the legaliza-
tion of marijuana for medical and recreational purposes, patients may be more
likely to ask physicians about its potential adverse and beneficial effects on
health. The popular notion seems to be that marijuana is a harmless pleasure, ac-
cess to which should not be regulated or considered illegal. Currently, marijuana is
the most commonly used “illicit” drug in the United States, with about 12% of
people 12 years of age or older reporting use in the past year and particularly high
rates of use among young people.
1
The most common route of administration is
inhalation. The greenish-gray shredded leaves and flowers of the Cannabis sativa
plant are smoked (along with stems and seeds) in cigarettes, cigars, pipes, water
pipes, or “blunts” (marijuana rolled in the tobacco-leaf wrapper from a cigar).
Hashish is a related product created from the resin of marijuana flowers and is
usually smoked (by itself or in a mixture with tobacco) but can be ingested orally.
Marijuana can also be used to brew tea, and its oil-based extract can be mixed into
food products.
The regular use of marijuana during adolescence is of particular concern, since
use by this age group is associated with an increased likelihood of deleterious
consequences
2
(Table 1). Although multiple studies have reported detrimental ef-
fects, others have not, and the question of whether marijuana is harmful remains
the subject of heated debate. Here we review the current state of the science re-
lated to the adverse health effects of the recreational use of marijuana, focusing
on those areas for which the evidence is strongest.
Adverse Effects
Risk of Addiction
Despite some contentious discussions regarding the addictiveness of marijuana,
the evidence clearly indicates that long-term marijuana use can lead to addiction.
Indeed, approximately 9% of those who experiment with marijuana will become
addicted3 (according to the criteria for dependence in the Diagnostic and Statistical
Manual of Mental Disorders, 4th edition [DSM-IV]). The number goes up to about 1 in
6 among those who start using marijuana as teenagers and to 25 to 50% among
those who smoke marijuana daily.4 According to the 2012 National Survey on Drug
Use and Health, an estimated 2.7 million people 12 years of age and older met the
DSM-IV criteria for dependence on marijuana, and 5.1 million people met the crite-
ria for dependence on any illicit drug1 (8.6 million met the criteria for dependence
on alcohol1). There is also recognition of a bona fide cannabis withdrawal syn-
drome5 (with symptoms that include irritability, sleeping difficulties, dysphoria,
craving, and anxiety), which makes cessation difficult and contributes to relapse.
Marijuana use by adolescents is particularly troublesome. Adolescents’ increased
vulnerability to adverse long-term outcomes from marijuana use is probably related
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to the fact that the brain, including the endocan-
nabinoid system, undergoes active development
during adolescence.6 Indeed, early and regular
marijuana use predicts an increased risk of mar-
ijuana addiction, which in turn predicts an in-
creased risk of the use of other illicit drugs.7 As
compared with persons who begin to use mari-
juana in adulthood, those who begin in adoles-
cence are approximately 2 to 4 times as likely to
have symptoms of cannabis dependence within
2 years after first use.8
Effect on Brain Development
The brain remains in a state of active, experi-
ence-guided development from the prenatal pe-
riod through childhood and adolescence until
the age of approximately 21 years.9 During these
developmental periods, it is intrinsically more
vulnerable than a mature brain to the adverse
long-term effects of environmental insults, such
as exposure to tetrahydrocannabinol, or THC,
the primary active ingredient in marijuana. This
view has received considerable support from
studies in animals, which have shown, for ex-
ample, that prenatal or adolescent exposure to
THC can recalibrate the sensitivity of the reward
system to other drugs10 and that prenatal expo-
sure interferes with cytoskeletal dynamics, which
are critical for the establishment of axonal con-
nections between neurons.11
As compared with unexposed controls, adults
who smoked marijuana regularly during adoles-
cence have impaired neural connectivity (fewer
fibers) in specific brain regions. These include
the precuneus, a key node that is involved in
functions that require a high degree of integra-
tion (e.g., alertness and self-conscious awareness),
and the fimbria, an area of the hippocampus
that is important in learning and memory.
12
Reduced functional connectivity has also been
reported in the prefrontal networks responsible
for executive function (including inhibitory con-
trol) and the subcortical networks, which pro-
cess habits and routines.
13
In addition, imaging
studies in persons who use cannabis have revealed
decreased activity in prefrontal regions and re-
duced volumes in the hippocampus.
14
Thus, cer-
tain brain regions may be more vulnerable than
others to the long-term effects of marijuana.
One study showed that selective down-regula-
tion of cannabinoid-1 (CB1) receptors in several
cortical brain regions in long-term marijuana
smokers was correlated with years of cannabis
smoking and was reversible after 4 weeks of
abstinence.
15
Changes in CB1 receptors were not
seen in subcortical regions.
The negative effect of marijuana use on the
functional connectivity of the brain is particu-
larly prominent if use starts in adolescence or
young adulthood,
12
which may help to explain
the finding of an association between frequent
use of marijuana from adolescence into adult-
hood and significant declines in IQ.
16
The im-
pairments in brain connectivity associated with
exposure to marijuana in adolescence are consis-
tent with preclinical findings indicating that the
cannabinoid system plays a prominent role in
synapse formation during brain development.
17
Possible Role as Gateway Drug
Epidemiologic and preclinical data suggest that
the use of marijuana in adolescence could inf lu-
ence multiple addictive behaviors in adulthood.
In rodents exposed to cannabinoids during ado-
lescence, there is decreased reactivity of the do-
pamine neurons that modulate the brain’s re-
ward regions.18 The exposure of rodents to
Table 1. Adverse Effects of Short-Term Use and Long-Term or Heavy Use
of Marijuana.
Effects of short-term use
Impaired short-term memory, making it difficult to learn and to retain infor-
mation
Impaired motor coordination, interfering with driving skills and increasing
the risk of injuries
Altered judgment, increasing the risk of sexual behaviors that facilitate the
transmission of sexually transmitted diseases
In high doses, paranoia and psychosis
Effects of long-term or heavy use
Addiction (in about 9% of users overall, 17% of those who begin use in ado-
lescence, and 25 to 50% of those who are daily users)*
Altered brain development*
Poor educational outcome, with increased likelihood of dropping out of school*
Cognitive impairment, with lower IQ among those who were frequent users
during adolescence*
Diminished life satisfaction and achievement (determined on the basis of
subjective and objective measures as compared with such ratings in the
general population)*
Symptoms of chronic bronchitis
Increased risk of chronic psychosis disorders (including schizophrenia) in
persons with a predisposition to such disorders
* The effect is strongly associated with initial marijuana use early in adolescence.
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cannabis in utero alters the developmental regu-
lation of the mesolimbic dopamine system of af-
fected offspring.19 If reduced dopamine reactivity
in the brain’s reward regions does follow early
exposure to marijuana, this effect could help to
explain the increased susceptibility to drug abuse
and addiction to several drugs later in life, which
has been reported in most epidemiologic stud-
ies.20 This theory is also consistent with animal
models showing that THC can prime the brain
for enhanced responses to other drugs.21 Al-
though these findings support the idea that mar-
ijuana is a gateway drug, other drugs, such as
alcohol and nicotine, can also be categorized as
gateway drugs, since they also prime the brain
for a heightened response to other drugs.22 How-
ever, an alternative explanation is that people
who are more susceptible to drug-taking behav-
ior are simply more likely to start with marijuana
because of its accessibility and that their subse-
quent social interactions with other drug users
would increase the probability that they would
try other drugs.
Relation to Mental Illness
Regular marijuana use is associated with an in-
creased risk of anxiety and depression,23 but cau-
sality has not been established. Marijuana is also
linked with psychoses (including those associat-
ed with schizophrenia), especially among people
with a preexisting genetic vulnerability,24 and
exacerbates the course of illness in patients with
schizophrenia. Heavier marijuana use, greater
drug potency, and exposure at a younger age can
all negatively affect the disease trajectory (e.g., by
advancing the time of a first psychotic episode by
2 to 6 years).25
However, it is inherently difficult to establish
causality in these types of studies because factors
other than marijuana use may be directly associ-
ated with the risk of mental illness. In addition,
other factors could predispose a person to both
marijuana use and mental illness. This makes it
difficult to confidently attribute the increased
risk of mental illness to marijuana use.
Effect on School Performance and Lifetime
Achievement
In the 2013 Monitoring the Future survey of
high-school students,26 6.5% of students in grade
12 reported daily or near-daily marijuana use,
and this figure probably represents an underesti-
mate of use, since young people who have
dropped out of school may have particularly high
rates of frequent marijuana use.27 Since marijua-
na use impairs critical cognitive functions, both
during acute intoxication and for days after use,28
many students could be functioning at a cogni-
tive level that is below their natural capability for
considerable periods of time. Although acute ef-
fects may subside after THC is cleared from the
brain, it nonetheless poses serious risks to health
that can be expected to accumulate with long-
term or heavy use. The evidence suggests that
such use results in measurable and long-lasting
cognitive impairments,16 particularly among
those who started to use marijuana in early ado-
lescence. Moreover, failure to learn at school,
even for short or sporadic periods (a secondary
effect of acute intoxication), will interfere with
the subsequent capacity to achieve increasingly
challenging educational goals, a finding that
may also explain the association between regular
marijuana use and poor grades.29
The relationship between cannabis use by
young people and psychosocial harm is likely to
be multifaceted, which may explain the incon-
sistencies among studies. For example, some
studies suggest that long-term deficits may be
reversible and remain subtle rather than dis-
abling once a person abstains from use.
30
Other
studies show that long-term, heavy use of mari-
juana results in impairments in memory and
attention that persist and worsen with increas-
ing years of regular use
31
and with the initiation
of use during adolescence.
32
As noted above,
early marijuana use is associated with impaired
school performance and an increased risk of
dropping out of school,
27,29
although reports of
shared environmental factors that inf luence the
risks of using cannabis at a young age and drop-
ping out of school
33
suggest that the relationship
may be more complex. Heavy marijuana use has
been linked to lower income, greater need for
socioeconomic assistance, unemployment, crim-
inal behavior, and lower satisfaction with life.
2,34
Risk of motor-vehicle Accidents
Both immediate exposure and long-term expo-
sure to marijuana impair driving ability; mari-
juana is the illicit drug most frequently reported
in connection with impaired driving and acci-
dents, including fatal accidents.35 There is a rela-
tionship between the blood THC concentration
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and performance in controlled driving-simula-
tion studies,36 which are a good predictor of real-
world driving ability. Recent marijuana smoking
and blood THC levels of 2 to 5 ng per milliliter
are associated with substantial driving impair-
ment.37 According to a meta-analysis, the overall
risk of involvement in an accident increases by a
factor of about 2 when a person drives soon after
using marijuana.37 In an accident culpability
analysis, persons testing positive for THC (typi-
cal minimum level of detection, 1 ng per milli-
liter), and particularly those with higher blood
levels, were 3 to 7 times as likely to be responsi-
ble for a motor-vehicle accident as persons who
had not used drugs or alcohol before driving.38
In comparison, the overall risk of a vehicular ac-
cident increases by a factor of almost 5 for drivers
with a blood alcohol level above 0.08%, the legal
limit in most countries, and increases by a factor
of 27 for persons younger than 21 years of age.39
Not surprisingly, the risk associated with the use
of alcohol in combination with marijuana ap-
pears to be greater than that associated with the
use of either drug alone.37
Risk of Cancer and Other Effects on Health
The effects of long-term marijuana smoking on
the risk of lung cancer are unclear. For example,
the use of marijuana for the equivalent of 30 or
more joint-years (with 1 joint-year of marijuana
use equal to 1 cigarette [joint] of marijuana
smoked per day for 1 year) was associated with
an increased incidence of lung cancer and several
cancers of the upper aerodigestive tract; however,
the association disappeared after adjustment for
potential confounders such as cigarette smok-
ing.40 Although the possibility of a positive asso-
ciation between marijuana smoking and cancer
cannot be ruled out,41 the evidence suggests that
the risk is lower with marijuana than with tobac-
co.40 However, the smoking of cigarettes that con-
tain both marijuana and tobacco products is a
potential confounding factor with a prevalence
that varies dramatically among countries.
Marijuana smoking is also associated with
inflammation of the large airways, increased
airway resistance, and lung hyperinflation, as-
sociations that are consistent with the fact that
regular marijuana smokers are more likely to
report symptoms of chronic bronchitis than are
nonsmokers
42
; however, the long-term effect of
low levels of marijuana exposure does not ap-
pear to be significant.
43
The immunologic com-
petence of the respiratory system in marijuana
smokers may also be compromised, as indicated
by increased rates of respiratory infections and
pneumonia.
44
Marijuana use has also been as-
sociated with vascular conditions that increase
the risks of myocardial infarction, stroke, and
transient ischemic attacks during marijuana in-
toxication.
45
The actual mechanisms underlying
the effects of marijuana on the cardiovascular
and cerebrovascular systems are complex and
not fully understood. However, the direct effects
of cannabinoids on various target receptors (i.e.,
CB1 receptors in arterial blood vessels) and the
indirect effects on vasoactive compounds
46
may
help explain the detrimental effects of marijua-
na on vascular resistance and coronary microcir-
culation.
47
Limitations of the Evidence
and Gaps in Knowledge
Most of the long-term effects of marijuana use
that are summarized here have been observed
among heavy or long-term users, but multiple
(often hidden) confounding factors detract from
our ability to establish causality (including the
frequent use of marijuana in combination with
other drugs). These factors also complicate our
ability to assess the true effect of intrauterine
exposure to marijuana. Indeed, despite the use
of marijuana by pregnant women,48 and animal
models suggesting that cannabis exposure dur-
ing pregnancy may alter the normal processes
and trajectories of brain development,49 our un-
derstanding of the long-term effects of prenatal
exposure to marijuana in humans is very poor.
The THC content, or potency, of marijuana,
as detected in confiscated samples, has been
steadily increasing from about 3% in the 1980s
to 12% in 2012
50
(Fig. 1A). This increase in THC
content raises concerns that the consequences of
marijuana use may be worse now than in the
past and may account for the significant in-
creases in emergency department visits by per-
sons reporting marijuana use
51
(Fig. 1B) and the
increases in fatal motor-vehicle accidents.
35
This
increase in THC potency over time also raises
questions about the current relevance of the
findings in older studies on the effects of mari-
juana use, especially studies that assessed long-
term outcomes.
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Adverse Health Effects of Marijuana Use
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There is also a need to improve our under-
standing of how to harness the potential medi-
cal benefits of the marijuana plant without ex-
posing people who are sick to its intrinsic risks.
The authoritative report by the Institute of
Medicine, Marijuana and Medicine,
52
acknowledges
the potential benefits of smoking marijuana in
stimulating appetite, particularly in patients
with the acquired immunodeficiency syndrome
(AIDS) and the related wasting syndrome, and in
combating chemotherapy-induced nausea and
vomiting, severe pain, and some forms of spas-
ticity. The report also indicates that there is
some evidence for the benefit of using marijuana
to decrease intraocular pressure in the treatment
of glaucoma. Nonetheless, the report stresses
the importance of focusing research efforts on
the therapeutic potential of synthetic or pharma-
ceutically pure cannabinoids.
52
Some physicians
continue to prescribe marijuana for medicinal
purposes despite limited evidence of a benefit
(see box). This practice raises particular con-
cerns with regard to long-term use by vulnerable
populations. For example, there is some evi-
dence to suggest that in patients with symptoms
of human immunodeficiency virus (HIV) infec-
tion or AIDS, marijuana use may actually exac-
erbate HIV-associated cognitive deficits.
75
Simi-
THC in Marijuana Samples (%)
14
10
12
8
6
4
2
0
1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012
BDrug-Related Emergency Department Visits
APotency of THC
No. of Emergency Department
Visits (in thousands)
600
400
500
300
200
100
0
2004
2005
2006
2007
2008
2009
2010
2011
2004
2005
2006
2007
2008
2009
2010
2011
2004
2005
2006
2007
2008
2009
2010
2011
Marijuana Cocaine Heroin
In combination Alone
123
135
258
182
323
505
129
327
456
Figure 1. Increases over Time in the Potency of Tetrahydrocannabinol (THC) in Marijuana and the Number of Emer-
gency Department Visits Involving Marijuana, Cocaine, or Heroin.
Panel A shows the increasing potency of marijuana (i.e., the percentage of THC) in samples seized by the Drug En-
forcement Administration (DEA) between 1995 and 2012.
50
Panel B provides estimates of the number of emergency
department visits involving the use of selected illicit drugs (marijuana, cocaine, and heroin) either singly or in com-
bination with other drugs between 2004 and 2011.
51
Among these three drugs, only marijuana, used either in com-
bination with other drugs or alone, was associated with significant increases in the number of visits during this peri-
od (a 62% increase when used in combination with other drugs and a 100% increase when used alone, P<0.05 for
the two comparisons).
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larly, more research is needed to understand the
potential effects of marijuana use on age-related
cognitive decline in general and on memory
impairment in particular.
Research is needed on the ways in which
government policies on marijuana affect public
health outcomes. Our understanding of the ef-
fects of policy on market forces is quite limited
(e.g., the allure of new tax-revenue streams from
the legal sale of marijuana, pricing wars, youth-
targeted advertising, and the emergence of can-
nabis-based medicines approved by the Food and
Drug Administration), as is our understanding
of the interrelated variables of perceptions about
Clinical Conditions with Symptoms That May Be Relieved by Treatment with Marijuana or Other Cannabinoids.*
Glaucoma
Early evidence of the benefits of marijuana in patients with glaucoma (a disease associated with increased pressure in
the eye) may be consistent with its ability to effect a transient decrease in intraocular pressure,
53,54
but other, stan-
dard treatments are currently more effective. THC, cannabinol, and nabilone (a synthetic cannabinoid similar to
THC), but not cannabidiol, were shown to lower intraocular pressure in rabbits.
55,56
More research is needed to es-
tablish whether molecules that modulate the endocannabinoid system may not only reduce intraocular pressure
but also provide a neuroprotective benefit in patients with glaucoma.
57
Nausea
Treatment of the nausea and vomiting associated with chemotherapy was one of the first medical uses of THC and other
cannabinoids.
58
THC is an effective antiemetic agent in patients undergoing chemotherapy,
59
but patients often state
that marijuana is more effective in suppressing nausea. Other, unidentified compounds in marijuana may enhance
the effect of THC (as appears to be the case with THC and cannabidiol, which operate through different antiemetic
mechanisms).
60
Paradoxically, increased vomiting (hyperemesis) has been reported with repeated marijuana use.
AIDS-associated anorexia and wasting syndrome
Reports have indicated that smoked or ingested cannabis improves appetite and leads to weight gain and improved mood
and quality of life among patients with AIDS.
61
However, there is no long-term or rigorous evidence of a sustained
effect of cannabis on AIDS-related morbidity and mortality, with an acceptable safety profile, that would justify its
incorporation into current clinical practice for patients who are receiving effective antiretroviral therapy.
62
Data from
the few studies that have explored the potential therapeutic value of cannabinoids for this patient population are
inconclusive.
62
Chronic pain
Marijuana has been used to relieve pain for centuries. Studies have shown that cannabinoids acting through central
CB1 receptors, and possibly peripheral CB1 and CB2 receptors,
63
play important roles in modeling nociceptive re-
sponses in various models of pain. These findings are consistent with reports that marijuana may be effective in
ameliorating neuropathic pain,
64,65
even at very low levels of THC (1.29%).
66
Both marijuana and dronabinol, a
pharmaceutical formulation of THC, decrease pain, but dronabinol may lead to longer-lasting reductions in pain
sensitivity and lower ratings of rewarding effects.
67
Inflammation
Cannabinoids (e.g., THC and cannabidiol) have substantial antiinflammatory effects because of their ability to induce
apoptosis, inhibit cell proliferation, and suppress cytokine production.
68
Cannabidiol has attracted particular inter-
est as an antiinflammatory agent because of its lack of psychoactive effects.
58
Animal models have shown that can-
nabidiol is a promising candidate for the treatment of rheumatoid arthritis
58
and for inflammatory diseases of the
gastrointestinal tract (e.g., ulcerative colitis and Crohn’s disease).
69
Multiple sclerosis
Nabiximols (Sativex, GW Pharmaceuticals), an oromucosal spray that delivers a mix of THC and cannabidiol, appears
to be an effective treatment for neuropathic pain, disturbed sleep, and spasticity in patients with multiple sclerosis.
Sativex is available in the United Kingdom, Canada, and several other countries
70,71
and is currently being reviewed
in phase 3 trials in the United States in order to gain approval from the Food and Drug Administration.
Epilepsy
In a recent small survey of parents who use marijuana with a high cannabidiol content to treat epileptic seizures in their
children,
72
11% (2 families out of the 19 that met the inclusion criteria) reported complete freedom from seizures,
42% (8 families) reported a reduction of more than 80% in seizure frequency, and 32% (6 families) reported a re-
duction of 25 to 60% in seizure frequency. Although such reports are promising, insufficient safety and efficacy data
are available on the use of cannabis botanicals for the treatment of epilepsy.
73
However, there is increasing evidence
of the role of cannabidiol as an antiepileptic agent in animal models.
74
* AIDS denotes acquired immunodeficiency syndrome, CB1 cannabinoid-1 receptor, and CB2 cannabinoid-2 receptor,
HIV human immunodeficiency virus, and THC tetrahydrocannabinol.
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Adverse Health Effects of Marijuana Use
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use, types of use, and outcomes. Historically,
there has been an inverse correlation between
marijuana use and the perception of its risks
among adolescents (Fig. 2A). Assuming that this
inverse relationship is causal, would greater per-
missiveness in culture and social policy lead to
an increase in the number of young people who
are exposed to cannabis on a regular basis?
Among students in grade 12, the reported preva-
lence of regular marijuana smoking has been
steadily increasing in recent years and may soon
intersect the trend line for regular tobacco
smoking (Fig. 2B). We also need information
about the effects of second-hand exposure to
cannabis smoke and cannabinoids. Second-hand
exposure is an important public health issue in
the context of tobacco smoking, but we do not
have a clear understanding of the effects of
second-hand exposure to marijuana smoking.
76
Studies in states (e.g., Colorado, California, and
Washington) and countries (e.g., Uruguay, Por-
tugal, and the Netherlands) where social and
legal policies are shifting may provide important
data for shaping future policies.
Conclusions
Marijuana use has been associated with substan-
tial adverse effects, some of which have been de-
termined with a high level of confidence (Table 2).
Marijuana, like other drugs of abuse, can result
in addiction. During intoxication, marijuana can
interfere with cognitive function (e.g., memory
and perception of time) and motor function (e.g.,
coordination), and these effects can have detri-
mental consequences (e.g., motor-vehicle acci-
dents). Repeated marijuana use during adoles-
cence may result in long-lasting changes in brain
function that can jeopardize educational, profes-
sional, and social achievements. However, the ef-
fects of a drug (legal or illegal) on individual
health are determined not only by its pharmaco-
logic properties but also by its availability and
social acceptability. In this respect, legal drugs
Grade 12 Students (%)
60
40
30
10
50
20
0
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009
2011
2013
BReported Daily Use of Cigarettes or Marijuana
ACorrelation between Perceived Risk and Use
Past-yr use
of marijuana
Perceived risk
of marijuana
Grade 12 Students (%)
50
40
30
10
20
0
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009
2011
2013
Daily cigarette use
in previous 30 days
Daily marijuana use
in previous 30 days
Table 2. Level of Confidence in the Evidence for Adverse Effects of Marijuana
on Health and Well-Being.
Effect Overall Level
of Confidence*
Addiction to marijuana and other substances High
Abnormal brain development Medium
Progression to use of other drugs Medium
Schizophrenia Medium
Depression or anxiety Medium
Diminished lifetime achievement High
Motor vehicle accidents High
Symptoms of chronic bronchitis High
Lung cancer Low
* The indicated overall level of confidence in the association between marijuana
use and the listed effects represents an attempt to rank the strength of the
current evidence, especially with regard to heavy or long-term use and use
that starts in adolescence.
Figure 2. Use of Marijuana in Relation to Perceived
Risk and Daily Use of Tobacco Cigarettes or Marijuana
among U.S. Students in Grade 12, 1975–2013.
Panel A shows the inverse correlation between the per-
ception of the risk associated with marijuana use and
actual use. Perceived risk corresponds to the percent-
age of teenagers who reported that the use of marijuana
is dangerous. Panel B shows the percentage of students
who reported daily use of tobacco cigarettes or mari-
juana in the previous 30 days.
Data for both graphs are
from Johnston et al.
26
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n engl j med 370;23 nejm.org june 5, 2014
2226
(alcohol and tobacco) offer a sobering perspec-
tive, accounting for the greatest burden of dis-
ease associated with drugs77 not because they are
more dangerous than illegal drugs but because
their legal status allows for more widespread ex-
posure. As policy shifts toward legalization of
marijuana, it is reasonable and probably prudent
to hypothesize that its use will increase and that,
by extension, so will the number of persons for
whom there will be negative health consequences.
No potential conflict of interest relevant to this article was
reported.
Disclosure forms provided by the authors are available with
the full text of this article at NEJM.org.
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