. UKCIA Research Library

Exposing 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


Probably 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 potential.

Although 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.

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.

For 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.

A 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.

Lifetime Prevalence of Marijuana, High Schools Seniors, 1976-1994

Year Percentage
1976 52.8
1978 59.2
1980 60.3
1982 58.7
1984 54.9
1986 50.9
1988 47.2
1990 40.7
1992 32.6
1994 38.2

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 Project

Year Percentage
1981 2.28
1982 3.05
1983 3.23
1984 2.39
1985 2.82
1986 2.30
1987 2.93
1988 3.29
1989 3.06
1990 3.36
1991 3.36
1992 3.00
1993 3.32

Even 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.

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.

In 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.

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).

Since 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.

The 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.

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 immunological activators.

There 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 controls.

Marijuana has been said to interfere with the production of hormones associated with reproduction, causing possible infertility among adult users and delayed sexual development among adolescents.

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.

The 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.

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.

Numerous other studies have failed to find negative impacts from marijuana exposure.

In 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.

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.

In 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 smoke regimen."

While 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.

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.

National epidemiological surveys show that the large majority of people who have had experience with marijuana do not become regular users.

In 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.

A 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 month.

Of 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.

Indeed, when humans are allowed to control consumption, even high doses are not followed by adverse withdrawal symptoms.

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.

In 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.

For 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%.

For 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.

Of 24,000 marijuana mentions in 1992, more than 13,000 involved alcohol and nearly 10,000 involved cocaine.

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.

In 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 values" scales.

Field studies conducted in Jamaica, Costa Rica and Greece also found no evidence of an amotivational syndrome among marijuana-using populations.

In 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.

In one study lasting 94 days, marijuana had no significant impact on learning, performance or motivation.

In 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.

However, 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 work.

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.

To 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.

A 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.

A 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.

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 cocaine.

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.

For 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.

In 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.

Percentage of Marijuana Users Ever Trying Cocaine, High School Seniors, 1975-1992

Year Percentage
1975 19
1976 19
1977 20
1978 22
1979 25
1980 27
1981 28
1982 27
1983 28
1984 29
1985 31
1986 33
1987 30
1988 26
1989 23
1990 22
1991 22
1992 18

In 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.

Whereas 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.

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.

This 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

Following 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.

Since 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.

Marijuana Use Among Dutch Youth (ages 12-18)

Year ever used past month
1984 4.8% 2.3%
1988 8.0 3.1
1992 13.6 6.5

Marijuana Use Amonst American Youth (ages 12-17)

Year ever used past month
1985 23.2% 11.2%
1988 24.7 6.4
1993 11.7 4.9

Marijuana Use Amongst American Youth (High School seniors)

Year ever used past month
1985 54.2% 25.7%
1988 47.2 18.0
1992 35.3 15.6

While 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.

Although 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.