IN THE CLASSIC 1936 cult film Reefer Madness, well-adjusted high school students who try marijuana suddenly sink into a life of addiction, promiscuity, aggression, academic failure, homicide and mental illness. The movie concludes with the ominous warning that “The dread marijuana may be reaching forth next for your son or daughter ... or yours ... or YOURS!” Newspaper headlines of the day often reflected a similar sentiment. On February 10, 1938, a headline in the Beloit (Wisc.) Daily News read, “Authorities Warn against Spread of Marijuana Habit—Insanity, Degeneracy and Violence Follow Use of Weed.”

Such a position on pot seems extreme. Yet just as people have since cast aside the notion that marijuana use inevitably culminates in the destruction of the mind, so have they also begun to question the concept that it is benign. In particular, some evidence suggests that marijuana can, in some cases, be addictive and that it may present other health problems as well, particularly in heavy users. That said, most people suffer no ill effects from a single or occasional use of the drug.

How Many Get Hooked?

Marijuana, which is also known as cannabis, is the most widely used illicit substance in the world, according to a United Nations report from 2002. Recreational use is widespread in the U.S., and medical use is on the rise. In a 2007 study psychologist Louisa Degenhardt of Michigan State University and her colleagues found that 43 percent of U.S. adults aged 18 or older have tried marijuana at least once. Many adolescents are drawn to the drug as well. In the large, ongoing Monitoring the Future study, researchers at the University of Michigan found that 14 percent of eighth graders had used marijuana at least once in the previous year with the number increasing to 35 percent for 12th graders. Marijuana use will undoubtedly grow in the near future because 16 states have already legalized it for medical use, and many more are considering legislation that would make it legal.

Given the drug’s growing popularity, many people have long been concerned about its potential dangers and, in particular, whether it can be addictive. People tend to use “addiction” and “dependence” interchangeably, although drug experts now favor the term “dependence.” In the current version of the mental health profession’s “bible,” the Diagnostic and Statistical Manual of Mental Disorders, a diagnosis of cannabis dependence (a type of substance dependence) requires a person to meet at least three of seven criteria.

A number of investigators have addressed this issue and found that only a relatively small percentage of those who try marijuana will become addicted. For example, in a large-scale survey published in 1994 epidemiologist James Anthony, then at the National Institute on Drug Abuse, and his colleagues asked more than 8,000 people between the ages of 15 and 64 about their use of marijuana and other drugs. The researchers found that of those who had tried marijuana at least once, about 9 percent eventually fit a diagnosis of cannabis dependence. The corresponding figure for alcohol was 15 percent; for cocaine, 17 percent; for heroin, 23 percent; and for nicotine, 32 percent. So although marijuana may be addictive for some, 91 percent of those who try it do not get hooked. Further, marijuana is less addictive than many other legal and illegal drugs.

Possible Perils

A hotly debated issue is whether marijuana is a “gateway” drug, leading to the use of more dangerous substances. Many studies have found that most people who used other illicit drugs had, in fact, used marijuana first. Although results such as these are consistent with the gateway hypothesis, they do not prove that using marijuana causes the use of other drugs. Those who are drawn to marijuana may simply be predisposed to drug use in general, regardless of their exposure to pot. In addition, individuals often smoke cigarettes or drink alcohol before they latch on to marijuana. Should we also be asking whether nicotine and alcohol are gateway drugs?

Researchers have also demonstrated that heavy marijuana use can lead to increased tolerance and withdrawal symptoms when trying to stop. In addition, heavy use can contribute to respiratory and cardiovascular problems as well as impairments in short-term memory. Marijuana may also trigger certain disorders, such as schizophrenia, in vulnerable persons [see “A Mind in Danger,” by Victoria Costello], although researchers continue to debate the evidence on this issue. Finally, because marijuana is still illegal in most states and under federal law, people who possess or sell marijuana may face legal consequences.

On the other hand, marijuana has significant upsides for individuals with certain illnesses. In glaucoma patients, it can reduce the dangerously high eye pressure that can lead to vision loss. In addition, pot can provide relief from chronic pain, reduce nausea and vomiting from cancer chemotherapy, and limit the severe weight loss that results from AIDS and other diseases.

When a person does become addicted, several types of psychotherapy can help him or her kick the habit. One of the more effective types is a form of cognitive-behavior therapy (CBT) tailored to the addictive mind-set. Using CBT, therapists teach patients practical coping skills that lead to a change in behavior. They also try to modify the thoughts that contribute to a person’s addiction. Two faster treatments are motivational interviewing and the closely related motivational-enhancement therapy. The goal of these methods is to boost a person’s drive to stop or reduce their use of pot.

Unfortunately, relapse rates remain high for all addiction psychotherapies. In a study published in 2003 psychologist Brent A. Moore, now at Yale University, and his colleagues found that 41 percent of successfully treated marijuana addicts had relapsed within six months. Scientists are searching for ways to bring about long-term abstinence more consistently.

The public needs to be aware of the facts about marijuana so that it can dismiss fictions about the drug’s effects. Only by knowing when marijuana presents a real threat and when the risk is minimal can people properly weigh its dangers and benefits in specific situations. Both our health and sound social policy depend on it.