Cannabis and Health

In strict medical terms, marijuana is much safer than many foods we usually consume….Cannabis in its natural form is one of the safest therapeutic active substances known to man…

Health professionals both drugs and experts are concerned about the effects of marijuana consumption… Among the main health problems are:


  • Cancer, especially lung cancer related to smoking, as well as other health problems related to smoking.
  • Concern that marijuana, which affects neural functions, may cause brain damage and may be a contributing factor in the development of schizophrenia, or at least lead to some sort of reduced function in cognitive tests.
  • “Amotivational syndrome.” This is the supposed propensity of cannabis to dull ambition and motivation and lead to “slacker” behavior.
  • The “gateway theory,” the belief that marijuana users may be predisposed to use other illicit substances.
  • Marijuana use by pregnant women has also been a source of concern.
  • Finally, with increased usage of marijuana and its possible eventual legalization, there is also worry about a possible increase in automobile accidents due to stoned drivers behind the wheel.

An overview of health-related concerns follows.

Cannabis and Cancer

Cancer, in particular lung cancer, is among the chief cannabis-related concerns of medical professionals because smoking is the primary method of administration. In 2006, however, the largest study of its kind concluded that smoking marijuana—even regularly and heavily—does not lead to lung cancer. The findings “were against our expectations,” according to pulmonologist Donald Tashkin of the University of California at Los Angeles.

“We hypothesized that there would be a positive correlation between marijuana use and lung cancer, and that the association would be more positive with heavier use,” Dr. Tashkin said. “What we found instead was no association at all, and even a suggestion of some protective effect.”

Federal health and drug enforcement officials had widely used Tashkin’s previous work on marijuana to make the case that the drug is dangerous. Tashkin, who has studied marijuana for more than three decades, said while he still believes marijuana is potentially harmful, its cancer-causing effects appear to be of less concern than previously thought.

In addition, the “suggestion of some protective effect” against lung cancer from smoking marijuana has shown up in scientific trials other than Dr. Tashkin’s. University of Virginia researchers had already stumbled upon this unexpected property in 1974 but, after mentions in a few news reports at the time, there had been an absence of follow-up study. (Marijuana research had been ongoing in the United States at the time, but funding had generally been available only if the intent was to prove harm, not to investigate medical uses of the plant.) That’s why the startling possibility of cannabic anti-cancer properties didn’t become public again until Tashkin’s study, and until the pioneering work conducted by Dr. Raphael Mechoulam (the doctor who first isolated THC in 1964) in Israel and by Dr. Manuel Guzman in Spain in 2003.


The U.S. government had actually done a secret 1996 follow-up study on the 1974 Virginia findings, hoping to disprove them, but when its study confirmed the results of the research—showing that THC, when administered to mice, protected them against cancer malignancy—the government tried to ignore the results. But a draft copy of the study was leaked to AIDS Treatment News, a medical journal, and some media covered the story.

In 2003, no fewer than five scientific journals published articles indicating that cannabinoids were potential anti-cancer agents. Despite the findings, mainstream media coverage was almost non-existent. The New York Times, Washington Post, and Los Angeles Times ignored the findings.

In 2009, a study published in the journal Cancer Prevention reported that lifetime marijuana use is associated with a “significantly reduced risk” of head and neck squamous cell carcinoma (SCC). “After adjusting for potential confounders (including smoking and alcohol drinking), ten to 20 years of marijuana use was associated with a significantly reduced risk of head and neck squamous cell carcinoma,” the authors reported.

Even subjects who smoked marijuana and also consumed alcohol and tobacco (known high-risk agents for head and neck cancers) also experienced a reduced risk of cancer, seemingly supporting the notion that cannabis may help protect users from damage inflicted by more harmful substances.

Another 2009 study appeared to support a link between marijuana use and testicular cancer in young men. A media frenzy developed when the study was released, but other researchers pointed out the study did not show a direct link, it only suggested a possible one, and it was based on a small sampling of 369 men who filled out a questionnaire. Responses were evaluated based on “lifestyle choices” and were compared to answers supplied by a group of 979 cancer-free men with similar backgrounds. Another problem is that the questionnaire relied on self-reported use of marijuana, which, because it is an illegal drug, can skew answers.


Of course inhaling smoke of any kind is bound to have some adverse impact on the lungs. It is almost certainly safer to employ other routes of administration such as vaporization, edibles, and topicals.

Cannabis and Brain Damage

As a psychotropic drug, marijuana was suspected early on of causing brain damage, memory loss, cognitive impairment, and reduced productivity.

One early study reported brain damage in rhesus monkeys after six months of exposure to high concentrations of marijuana smoke; however, other researchers speculated any effects were due to oxygen deprivation and not cannabis. In a more recent, carefully controlled study, scientists found no evidence of brain abnormality in monkeys forced to inhale the equivalent of four to five joints every day for a year.


Other recent related studies have shown that cannabis stimulated the growth of new neurons, or brain cells, in the hippocampi of mice, and reduced measures of depression and anxiety seemingly related to the neurogeneses.

None of the medical tests currently used to detect brain damage in adult humans has found harm from marijuana, even from long-term, high-dose use. “We were somewhat surprised by our finding, especially since there’s been a controversy for some years on whether long-term cannabis use causes brain damage,” said lead researcher and psychiatrist Igor Grant. “I suppose we expected to see some differences in people who were heavy users,” Dr. Grant states, “but in fact the differences were very minimal.” All study participants were adults. Grant noted, “There might be a different set of circumstances to a 12-year-old whose nervous system is still developing.”

One small 2010 study including 35 young cannabis users (20 who’d started smoking marijuana regularly before age 16, and 15 who started at age 16 or later) concluded that heavy marijuana use during early adolescence causes greater brain function impairment than using cannabis later. The study indicated that regular pot use before the age of 16 leaves individuals with “weakened executive function,” which includes planning, cognitive flexibility, and abstract thinking. The brain is still changing at age 15, according to the study’s researchers, and the part of the brain associated with executive function is the last to develop.

Cannabis and Schizophrenia

Some studies show that cannabis use, particularly heavy use, by youths under the age of 21, is correlated with a greater risk of developing schizophrenia (again, as discussed above, because the brain is still developing).

Others speculate that perhaps youthful schizophrenics self-medicate with marijuana upon the onset of their symptoms, with the schizophrenia resulting in cannabis use rather than the other way around. This view receives support from survey data showing male patients diagnosed with schizophrenia report obtaining “subjective benefits” from using marijuana. Researchers reported people used marijuana “as a means of satisfying the schizophrenia-related need for relaxation, sense of self-worth, and distraction.”

A scientific review published in the journal Addiction assessed whether marijuana use may be causally linked with mental illness but found the evidence “overstated” and “not compelling.”


“We continue to take the view that the evidence that cannabis use causes schizophrenia is neither very new, nor by normal criteria, particularly compelling. . . . For example, our recent modeling suggests that we would need to prevent between 3,000 and 5,000 cases of heavy cannabis use among young men and women to prevent one case of schizophrenia, and that four or five times more young people would need to avoid light cannabis use to prevent a single schizophrenia case.”

The weakest part of any theory which posits a link between marijuana use and the incidence of schizophrenia may be that while cannabis went from being a rare secret shared by a small community in the 1940s and 1950s to a rite of passage for some 70 percent of youth by the turn of the 21st century, rates of schizophrenia in the U.S. have remained flat, or possibly even declined.

For as long as it has been tracked, schizophrenia has been found to affect only about one percent of the population. Additionally, gender alone may account for a large part of any supposed link. In men, schizophrenia tends to take hold around age 19; in women it isn’t typically seen until around age 22—whether or not marijuana use occurs. But typically, teenage boys are heavy pot smokers at a rate four times that of teenage girls, which may create an illusory “association” between marijuana use and onset of the disease.

A simple causative effect by which cannabis use may induce schizophrenia in otherwise normal individuals is hardly tenable,” concluded the authors of “Neuropsychiatry: Schizophrenia, Depression and Anxiety.


In 2011, however, researchers at the University of New South Wales in Sydney, Australia, reported findings demonstrating that when pot smokers become mentally ill, the disease presents earlier than it would if the users had not smoked marijuana. Marijuana users’ symptoms began about three years before non-users. Dr. Matthew Large, who headed up the research, said the findings add to “growing evidence” that pot use may cause psychotic disorders such as schizophrenia in some people. The researchers reported the link between pot use and developing mental illness is strongest in 12- to 15-year olds and children even younger. Dr. Large stressed that pre-teens and young adolescents should not be exposed to marijuana, in order to allow for those predisposed toward mental illness “an extra two or three years of psychosis-free functioning [that] could allow many . . . to achieve important developmental milestones.”

Cannabis and Amotivational Syndrome

Those favoring marijuana prohibition have for years alleged that cannabis tends to make regular users passive, lazy, and unmotivated. But the largest scientific study on marijuana and motivation—the Rubin Study in Jamaica, published in the 1970s by Dr. Vera Rubin—showed that Jamaican marijuana users worked even harder than non-using co-workers.


In other studies, people given high doses of marijuana for several days or even weeks have exhibited no decrease in work motivation or productivity. And among working adults, marijuana users tend to earn higher wages than non-users. College students who use marijuana have the same grades, on average, as non-users. Among high school students, heavy cannabis use is associated with school failure, but school failure may come first.

The Gateway Theory: Does Cannabis Lead to Hard Drugs?

Another concern with marijuana is the so-called gateway theory, which posits that marijuana use may open the door to abuse of stronger drugs.

Health professionals hold that the use of marijuana, especially early in life, may be a predictor of the likelihood of hard drug use later on. Because marijuana and hard drugs are obtained in underground markets, user interaction with dealers and others may lead to experimentation with drugs such as heroin, cocaine, and methamphetamine. But this is a result of social and environmental factors, not marijuana use itself. The medical community also reminds us that a percentage of marijuana users are likely to have a proclivity for dependence, whether on alcohol, tobacco, or other substances. (Marijuana does not introduce physical dependence as does alcohol or heroin, yet some users may find it psychologically difficult to stop.)

Many studies disprove the gateway theory. The 1944 “LaGuardia Report” (it was commissioned by New York City Mayor Fiorella LaGuardia), a six-year effort conducted by a team of scientists from the New York Academy of Medicine, was the most comprehensive and extensive cannabis study since the Indian Hemp Drug Commission released its findings about 50 years earlier. The study found: “The use of marijuana does not lead to morphine or heroin or cocaine addiction…. The instances are extremely rare where the habit of marihuana smoking is associated with addiction to these narcotics.”

The next major study, conducted by President Nixon’s Shafer Commission in 1972, had similar conclusions: “Incidence of other drug use was relatively low, [even among] frequent marihuana users.” Vera Rubin’s 1975 Jamaica Study noticed the same phenomenon. “One of the more interesting findings to emerge from this study relates to the ‘stepping-stone’ hypothesis. . . . Nothing like that occurs among heavy, chronic ganja smokers of Jamaica,” the study concluded. “No other drugs were used, aside from aspirin, tea, alcohol, and tobacco. The only hard drug use known on the island is indulged by North American tourists.”


A fourth federal study reaffirmed these findings a few years later. Released in 1982, the 15-month study, conducted by the Institute of Medicine of the National Academy of Sciences, concluded: “There is no evidence to support the belief that the use of one drug will inevitably lead to the use of any other drug.”

One of the most compelling arguments against the gateway theory is the fact that it doesn’t hold up in the face of the federal government’s own figures regarding drug use. For instance, as marijuana use increased in the 1960s and 1970s, heroin use declined; while cocaine use increased in the 1980s, marijuana use fell. Federal statistics overall indicate that American marijuana users do not move on to harder drugs.

More recently, the Advisory Council on the Misuse of Drugs (2002) reported that the overwhelming majority of marijuana users never try any other illicit substance.

Cannabis and Pregnancy

Women should consult their doctors about the risks of taking any drug (over-the-counter or otherwise) or dietary supplement when pregnant or breast-feeding. Research regarding the effects of marijuana on unborn children is unclear; however, THC does cross the placenta and may affect the developing fetus.