Marijuana is the most widely used illicit drug in the country -- an estimated 25 million Americans smoked it within the last year and close to 100 million have smoked it at least once in their life, according to the most recent National Survey on Drug Use and Health by the federal Substance Abuse and Mental Health Services Administration.
Rates and severity of marijuana addiction pale in comparison to that of legal addictive drugs, alcohol and nicotine, according to the Advisory Council on the Misuse of Drugs, a panel of independent experts advising the British government, in a rare head-to-head, scientific comparison.
Yet, the fact is, recreational use can lead to addiction, and inhaling marijuana smoke is unhealthful for the lungs. Some researchers argue that marijuana may predispose heavy users to mental illnesses such as psychosis and depression.
How big are these risks and how should they be measured against health benefits? "The FDA has ruled that marijuana has no medical benefits, but its harms are well known and proven," says Tom Riley, a spokesman for the White House Office of National Drug Control Policy, referring to an April 2006 statement released by the FDA and several other federal agencies concluding that smoking marijuana was not of medicinal use.
For comparison's sake, Riley cites the prescription drug Vioxx. The FDA, he notes, pulled Vioxx off the market in spite of its proven efficacy, because it created problems in a small number of people.
Then, too, the number of people adversely affected by marijuana use is large, Riley says. "There are more teens in drug treatment for marijuana dependence than for alcohol or any other drug," he says.
Marijuana is a Schedule 1 drug by the Drug Enforcement Administration's Controlled Substances Act, a classification reserved for drugs carrying the highest risk for addiction and no medical benefit.
Scientists have reviewed the weed's risks and find them to be real, but small. Ten years ago, the Institute of Medicine reviewed the scientific evidence about marijuana at the request of the Office of National Drug Control Policy. The 1999 report states that, "except for the harms associated with smoking, the adverse effects of marijuana use are within the range of effects tolerated for other medications."
In February, the American College of Physicians, the nation's second-largest physicians group, released a position paper in support of medical-marijuana research, protecting doctors from criminal prosecution and rescheduling marijuana as a less harmful drug.
A British advisory group this year found no evidence to reclassify cannabis as a more harmful drug in that country. In contrast to the U.S., the U.K. puts cannabis in the lowest category (Class C) in terms of criminal penalties for possession or sale, although government officials are campaigning to move it to Class B.
To investigate the risks of marijuana, researchers typically use heavy marijuana smokers as subjects. Though such a study design may be convenient, it makes interpretation tricky because heavy users may have traits in common besides smoking pot. Thus, says psychologist and marijuana researcher Stanley Zammit of Cardiff University in Wales, it is not easy in these kinds of studies to separate out the contribution of marijuana to any measurable effect in the group.
Claims of a link between marijuana use and psychotic episodes came under scrutiny after the U.K. downgraded cannabis from Class B to Class C in 2004. In 2007, Zammit was asked by England's Department of Health to survey the existing evidence to determine the long-term risks for mental illness from using cannabis. After researching the literature and including only those studies that satisfied certain criteria, he combined the results in a 2007 Lancet paper.
He concluded that marijuana use was associated with an increased risk of psychosis -- ranging from self-reported symptoms such as delusions or hallucinations to clinically diagnosed schizophrenia.
The risk is small, he adds. Cannabis use was associated with a 40% increase in risk overall and up to a twofold increase in heavy users. Because the risk of any person developing psychosis in their lifetime is about 2% to 3%, cannabis use at worst increases that to 5%. "So 95% of the people are not going to get psychotic, even if they smoke on a daily basis," Zammit says.
Zammit adds that "the main limitations of these studies is that you can never be sure that it's the cannabis itself that's causing this risk." Heavy users of marijuana may differ from nonusers in other traits -- including those that lead independently to increased drug use and risk of psychosis. The studies he reviewed tried to take into account this possibility but could not rule it out entirely.
The bottom line? "The evidence is probably strong enough that people should be aware of this risk," he says.