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MARIJUANA MYTHS: A REVIEW OF THE SCIENTIFIC EVIDENCE
Lynn Zimmer, Associate Professor of Sociology Queens College
and John P.
Morgan, Professor of Pharmacology, City University Medical School
the most important studies of the 1970s were three large "field studies" in Greece,
Costa Rica and Jamaica. These studies, which evaluated the impact of marijuana
on users in their natural environments, were supplemented by clinical examinations
and laboratory experiments oriented toward answering the questions about marijuana
that continued to be debated in the scientific literature. The data from these
studies, published in numerous books and scholarly journals, covered such matters
as marijuana's effects on the brain, lungs, immune and reproductive systems, its
impact on personality, development, and motivational states, and its addictive
these studies did not answer all remaining questions about marijuana toxicity,
they generally supported the idea that marijuana was a relatively safe drug --
not totally free from potential harm, but unlikely to create serious harm for
most individual users or society. In the years since, thousands of additional
studies have been conducted, many of them funded by NIDA, and together they reaffirm
marijuana's substantial margin of safety. Our review of that body of work reveals
an occasional study indicating greater toxicity than previously thought. But in
nearly all such cases, the methodologies were seriously flawed and the findings
could not be replicated by other researchers.
#1: MARIJUANA USE IS INCREASING AT AN ALARMING RATE
Reports of a recent slight
increase in marijuana use, especially among youth, are being used to convince
Americans that a renewed campaign about the drug's dangers is necessary to avert
an impending epidemic.
According to government surveys of the general population, marijuana
use began decreasing in 1980, after more than a decade of steady increase. By
1990, the downward trend showed signs of slowing, but use-rates remained substantially
lower than those recorded in the 1970s.
example, among 12-17 year olds, past year marijuana use was about 8 percent in
1992, compared to 24.1 percent in 1979. Among 18-25 year olds, past year use was
23 percent in 1992, compared to 46.9 percent in 1979.
separate survey of high school students shows similar trends, with use-rates in
the 1990s well below those reported in the 1970s. However, after reaching an all-time
low in 1992, they increased slightly during the next two years.
Prevalence of Marijuana, High Schools Seniors, 1976-1994
#2: MARIJUANA POTENCY HAS INCREASED SUBSTANTIALLY
The claim that there has
been a 10-, 20- or 30-fold increase in marijuana potency since the 1970s is used
to discredit previous studies that showed minimal harm caused by the drug and
convince users from earlier eras that today's marijuana is much more dangerous.
Mean Percentage THC of Seized Marijuana, 1981-1993, Mississippi Monitoring
if potency had increased slightly since the 1970s, it would not mean that smoking
marijuana had become more dangerous. In fact, since the primary health risk of
marijuana comes from smoking, higher potency products can be less dangerous because
they allow people to achieve the desired effect by inhaling less.
#3: MARIJUANA IS A DRUG WITHOUT THERAPEUTIC VALUE
Proposals to make marijuana
legally available as a medicine are countered with claims that safer, more effective
drugs are available, including a synthetic version of delta-9-THC, marijuana's
primary active ingredient.
People undergoing cancer chemotherapy have found smoked marijuana to
be an effective anti-nauseant-often more effective than available pharmaceutical
medications. Indeed, 44 percent of oncologists responding to a questionnaire said
they had recommended marijuana to their cancer patients; others said they would
recommend it if it were legal.
1986, a synthetic delta-9-THC capsule (Marinol) was marketed in the United States
and labeled for use as an anti-emetic. Despite some utility, this product has
serious drawbacks, including its cost. For example, a patient taking three 5 mgm
capsules a day would spend over $5,000 to use Marinol for one year. In comparison
to the natural, smokeable product Marinol also has some pharmacological shortcomings.
Because THC delivered in oral capsules enters the bloodstream slowly, it yields
lower serum concentrations per dose. Oral THC circulates in the body longer at
effective concentrations, and more of it is metabolized to an active compound;
thus, it more frequently yields unpleasant psychoactive effects. In patients suffering
from nausea, the swallowing of capsules may itself provoke vomiting. In short,
the smoking of crude marijuana is more efficient in delivering THC and, in some
cases, it may be more effective.
#4: MARIJUANA CAUSES LUNG DISEASE
It is frequently claimed that marijuana smoke
contains such high concentrations of irritants that marijuana users' risk of developing
lung disease is equal to or greater than that of tobacco users.
Frequent marijuana smokers experience adverse respiratory symptoms from
smoking, including chronic cough, chronic phlegm, and wheezing. However, the only
prospective clinical study shows no increased risk of crippling pulmonary disease
(chronic bronchitis and emphysema).
1982, UCLA researchers have evaluated pulmonary function and bronchial cell characteristics
in marijuana-only smokers, tobacco-only smokers, smokers of both, and non-smokers.
Although they have found changes in marijuana-only smokers, the changes are much
less pronounced than those found in tobacco smokers.
nature of the marijuana-induced changes were also different, occuring primarily
in the lung's large airways-not the small peripheral airways affected by tobacco
smoke. Since it is small-airway inflamation that causes chronic bronchitis and
emphysema, marijuana smokers may not develop these diseases.
#5: MARIJUANA IMPAIRS IMMUNE SYSTEM FUNCTIONING
It has been widely claimed
that marijuana substantially increases users' risk of contracting various infectious
diseases. First emerging in the 1970s, this claim took on new significance in
the 1980s, following reports of marijuana use by people suffering from AIDS.
The principal study fueling the original claim of immune impairment
involved preparations created with white blood cells that had been removed from
marijuana smokers and controls. After exposing the cells to known immune activators,
researchers reported a lower rate of "transformation" in those taken from marijuana
smokers. However, numerous groups of scientists, using similar techniques, have
failed to confirm this original study. In fact, a 1988 study demonstrated an increase
in responsiveness when white blood cells from marijuana smokers were exposed to
have been no clinical or epidemiological studies showing an increase in bacterial,
viral, or parasitic infection among human marijuana users. In three large field
studies conducted in the 1970s, in Jamaica, Costa Rica and Greece, researchers
found no differences in disease susceptibility between marijuana users and matched
#6: MARIJUANA HARMS SEXUAL MATURATION AND REPRODUCTION
Marijuana has been
said to interfere with the production of hormones associated with reproduction,
causing possible infertility among adult users and delayed sexual development
There is no evidence that marijuana impairs male reproductive functioning.
The Jamaican and Costa Rican field studies detected no differences in hormone
levels between marijuana users and non- users. In epidemiological surveys of marijuana
users, no problems with fertility have emerged as important.
claim that marijuana impairs female reproductive functioning in humans has no
support in the scientific literature. There have been no epidemiological studies
indicating diminished fertility in female users of marijuana, and a recent survey
found no impact of chronic marijuana use on female sex hormones.
#7: MARIJUANA USE DURING PREGNANCY HARMS THE FETUS
A powerful accusation in
anti-drug campaigns is that children are permanently harmed by their mothers'
use of drugs during pregnancy. Today, it is commonly claimed that marijuana is
a cause of birth defects and development deficits.
A number of studies reported low birth weight and physical abnormalities
among babies exposed to marijuana in utero. However, when other factors known
to affect pregnancy outcomes were controlled for-for example, maternal age, socio-economic
class, and alcohol and tobacco use-the association between marijuana use and adverse
fetal effects disappeared.
other studies have failed to find negative impacts from marijuana exposure.
another study, standardized IQ tests were administered to marijuana- exposed and
unexposed 3 year-olds. Researchers found no differences in the overall scores.
However, by dividing the sample by race, they found- among African-American children
only-lower scores on one subscale for those exposed during the first trimester
and lower scores on a different subscale for those exposed during the second trimester.
#8: MARIJUANA CAUSES BRAIN DAMAGE
Critics state that marijuana damages brain
cells and that this damage, in turn, causes memory loss, cognitive impairment,
and difficulties in learning.
The original basis of this claim was a report that, upon post-mortem
examination, structural changes in several brain regions were found in two rhesus
monkeys exposed to THC. However, to achieve these results, massive doses of THC-up
to 200 times the psychoactive dose in humans-had to be given. In fact, studies
employing 100 times the human dose have failed to reveal any damage.
the most recently published study, rhesus monkeys, through face-mask inhalation,
were exposed to the equivalent of 4-5 joints per day for an entire year. When
sacrificed seven months later, there was no observed alteration of hippocampal
architecture, cell size, cell number, or synaptic configuration. The authors conclude
that: "while behavioral and neuroendocrinal effects were observed during marijuana
smoke exposure in the monkey, residual neuropathological and neurochemical effects
of marijuana exposure were not observed seven months after the year-long marijuana
there is general agreement that, while under the influence of marijuana, learning
is less efficient, there is no evidence that marijuana users-even long-term users-suffer
permanent impairment. Indeed, numerous studies comparing chronic marijuana users
with non-user controls have found no significant differences in learning, memory
recall, or other cognitive functions.
#9: MARIJUANA IS AN ADDICTIVE DRUG
It is now frequently stated that marijuana
is profoundly addicting and that any increase in prevalence of use will lead inevitably
to increases in addiction.
Essentially all drugs are used in "an addictive fashion" by some people.
However, for any drug to be identified as highly addictive, there should be evidence
that substantial numbers of users repeatedly fail in their attempts to discontinue
use and develop use-patterns that interfere with other life activities.
epidemiological surveys show that the large majority of people who have had experience
with marijuana do not become regular users.
1993, among Americans age 12 and over, about 34% had used marijuana sometime in
their life, but only 9% had used it in the past year, 4.3% in the past month,
and 2.8% in the past week.
longitudinal study of young adults who had first been surveyed in high school
also found a high "discontinuation rate" for marijuana. While 77% had used the
drug, 74% of those had not used in the past year and 84% had not used in the past
course, even people who continue using marijuana for several years or more are
not necessarily "addicted" to it. Many regular users-including many daily users-consume
marijuana in a way that does not interfere with other life activities, and may
in some cases enhance them. There is only scant evidence that marijuana produces
physical dependence and withdrawal in humans.
when humans are allowed to control consumption, even high doses are not followed
by adverse withdrawal symptoms.
#10: MARIJUANA-RELATED MEDICAL EMERGENCIES ARE INCREASING
As evidence of its
harmful effects, prohibition advocates point to dramatic increases in emergency-room
episodes related to marijuana ingestion.
Despite marijuana being the most frequently used illicit drug, in emergency
rooms, it remains the least often mentioned illicit drug.
1993, marijuana accounted for 6.25% of mentions, compared to 15.3% for cocaine
and 9.8% for heroin. Even over-the-counter pain medications were mentioned more
often than marijuana-comprising 9% of the total.
youth aged 6 to 17, there were more mentions of marijuana than of heroin and cocaine-not
because marijuana is more harmful to them but because these latter drugs are used
so infrequently by young people. In this age group, mentions of over-the-counter
pain medications were substantially higher than those for marijuana. While marijuana
accounted for 6.48% of drug mentions by youth, over-the- counter pain medications
accounted for 47%.
the total population, not only is marijuana mentioned less frequently than other
recreational drugs, it is seldom mentioned alone. In 1992, in more than 80% of
the drug-abuse episodes involving marijuana, at least one other drug was mentioned;
and, in more than 40%, two or more additional drugs were mentioned.
24,000 marijuana mentions in 1992, more than 13,000 involved alcohol and nearly
10,000 involved cocaine.
#11: MARIJUANA PRODUCES AN AMOTIVATIONAL SYNDROME
Marijuana is said to have
a deliterious effect on society by making users passive, apathetic, unproductive,
and unable (or unwilling) to fulfill their responsibilities.
Large-scale studies of high school students have generally found no difference
in grade-point averages between marijuana users and non-users. One study found
lower grades among students reported to be daily users of marijuana, but the authors
failed to identify a causal relationship and concluded that both phenomena were
part of a complex of inter-related social and emotional problems.
one longitudinal study of college students, after controlling for other factors,
marijuana users were found to have higher grades than non-users and to be equally
as likely to successfully complete their educations. Another study found that
marijuana users in college scored higher than non-users on standardized "achievement
studies conducted in Jamaica, Costa Rica and Greece also found no evidence of
an amotivational syndrome among marijuana-using populations.
these samples of working-class males, the educational and employment records of
marijuana users were, for the most part, similar to those of non-users. In fact,
in Jamaica, marijuana was often smoked during working hours as an aid to productivity.
The results of laboratory studies have been nearly as consistent.
one study lasting 94 days, marijuana had no significant impact on learning, performance
another 31-day study, subjects given marijuana worked more hours than controls
and turned in an equal number of tokens for cash at the study's completion.
the weight of scientific evidence suggests that there is nothing in the pharmacological
properties of cannabis to alter people's attitudes, values, or abilities regarding
#12: MARIJUANA IS A MAJOR CAUSE OF HIGHWAY ACCIDENTS
The detrimental impact
of alcohol on highway safety has been well documented. Marijuana's opponents claim
that it, too, causes significant impairment and that any increase in use will
lead to increased highway accidents and fatalities.
In high doses, marijuana probably produces driving impairment in most
people. However, there is no evidence that marijuana, in current consumption patterns,
contributes substantially to the rate of vehicular accidents in America.
accurately assess marijuana's contribution to fatal crashes, the positive rate
among deceased drivers would have to be compared to the positive rate from a random
sample of drivers not involved in fatal accidents. Since the rate of past-month
marijuana use for Americans above the legal driving age is about 12 percent, on
any given day a substantial proportion of all drivers would test positive, particularly
since marijuana's metabolites remain in blood and urine long after its psychoactive
effects are finished.
number of driving simulator studies have shown that marijuana does not produce
the kind of psychomotor impairment evident with modest doses of alcohol. In fact,
in a recent NHTSA study, the only statistically significant outcome associated
with marijuana was speed reduction.
recent study of actual driving ability under the influence of cannabis- employing
the same protocol used to test the impairment-potential of medicinal drugs-evaluated
the impact of placebo and three active THC doses in three driving trials, including
one in high-density urban traffic.
#13: MARIJUANA IS A "GATEWAY" TO THE USE OF OTHER DRUGS
Advocates of marijuana
prohibition claim that even if marijuana itself causes minimal harm, it is a dangerous
substance because it leads to the use of "harder drugs" such as heroin, LSD, and
Most users of heroin, LSD and cocaine have used marijuana. However, most
marijuana users never use another illegal drug. Over time, there has been no consistent
relationship between the use patterns of various drugs. As marijuana use increased
in the 1960s and 1970s, heroin use declined. And, when marijuana use declined
in the 1980s, heroin use remained fairly stable.
the past 20 years, as marijuana use-rates fluxuated, the use of LSD hardly changed
at all. Cocaine use increased in the early 1980s as marijuana use was declining.
During the late 1980s, both marijuana and cocaine declined. During the last few
years, cocaine use has continued to decline as marijuana use has increased slightly.
1994, less than 16 percent of high school seniors who had ever tried marijuana
had ever tried cocaine-the lowest percentage ever recorded. In fact, as shown
below, the proportion of marijuana users trying cocaine has declined steadily
since 1986, when a high of more than 33 percent was recorded.
of Marijuana Users Ever Trying Cocaine, High School Seniors, 1975-1992
short, there is no inevitable relationship between the use of marijuana and other
drugs. This fact is supported by data from other countries. In Holland, for example,
although marijuana prevalence among young people increased during the past decade,
cocaine use decreased-and remains considerably lower than in the United States.
approximately 16 percent of youthful marijuana users in the U.S. have tried cocaine,
the comparable figure for Dutch youth is 1.8 percent. Indeed, Holland's policy
of allowing marijuana to be purchased openly in government-regulated "coffee shops"
was designed specifically to separate young marijuana users from illegal markets
where heroin and cocaine are sold.
#14: DUTCH MARIJUANA POLICY HAS BEEN A FAILURE
While American critics of marijuana
prohibition often point to Holland as a model for an alternative policy, prohibition's
supporters claim that Holland's permissiveness has had disastrous consequences,
including escalating rates of drug use among youth.
In 1976, following the recommendations of two national commissions, the
Dutch government revised many aspects of its drug policy. While not legalizing
marijuana, it adopted an "expediency principle," which directed police and prosecutors
to ignore retail sale to adults as long as the circumstances of the sale do not
constitute a public nuisance.
change in policy was based on several factors, including: a principle of tolerance
toward alternative lifestyles a finding that, compared to other illegal drugs,
marijuana poses little risk to users a desire to protect marijuana users from
the marginalization that accompanies arrest and prosecution a belief that separating
the retail markets for "soft" and "hard" drugs decreases the likelihood that marijuana
users will experiment with cocaine or heroin
the policy change, marijuana sales emerged openly in coffee shops, which were
required to follow a set of regulations, including a ban on advertising, sale
of no more than 30 grams (now 5 grams) at a time, and a minimum purchase age of
18. The sale of other drugs on the premises is strictly prohibited, and constitutes
grounds for immediate closure by the police. Local officials were also authorized
to create additional regulations to protect the interests of the community-for
example, limiting the number of coffee shops concentrated in any one area.
liberalization, marijuana use has increased in the Netherlands, although rates
remain similar to those in neighboring European countries, and are generally lower
than those in the United States.
Use Among Dutch Youth (ages 12-18)
Use Amonst American Youth (ages 12-17)
Use Amongst American Youth (High School seniors)
marijuana rates have increased in Holland, cocaine use-rates have not - indicating
that separation of the "hard" and "soft" drug markets has prevented a "gateway
effect" from developing. In 1992, about 1.5% of 12 to 18 year-olds had ever tried
cocaine and only 0.3% had used it in the past month.
there are some Dutch critics of Holland's liberalized marijuana policy, the government's
official position remains steadfastly supportive of the 1976 initiative that decriminalised
possession and retail sale.