Even if it's real, the risk of developing psychosis because of marijuana use is smaller than with use of some other drugs -- including legal ones such as cigarettes, says Mitch Earleywine, a psychologist at the State University of New York University at Albany.
Grant says that numbers of schizophrenia cases have not increased since before the 1960s, when widespread marijuana use began. "The data are variable to be sure, but most studies have found that over the years the rate of schizophrenia has been stable or even declining," he says.
In an American Journal of Psychiatry study, 1,920 adults were assessed for marijuana use and depression and followed for 15 years. In those subjects who had no depressive symptoms at the study's start, marijuana abusers were four times more likely to develop depressive symptoms down the road. But Zammit, who reviewed this paper and 23 others in his 2007 Lancet paper, says the data overall are even murkier than for psychosis. Most of the studies he reviewed did not assess symptoms of depression before marijuana use, and so didn't rule out the idea that depression makes someone more likely to smoke marijuana -- and not the other way around.
A review of the scientific literature published in the Journal of the International Neuropsychological Society in 2003 looked at whether marijuana smoking had lasting effects on cognition after THC has left the body. Marijuana use was found to have small effects on memory in long-term users -- measured by asking subjects to recall words, for instance -- but no differences were seen on attention, verbal skills and reaction time. "We were actually surprised," says Grant, an author on the study. Even if the marijuana itself wasn't causing such things, he expected marijuana users might have other less-than-healthful behaviors -- they may drink a bit more, or use some other drugs, and "you might expect them to do a little worse."
A 2002 study published in the Journal of the American Medical Assn. found that a group of 51 heavy marijuana users (two joints per day) recalled two to three fewer words on average than nonusers in a memory test with a list of 15 words.
A second study, published in the Archives of General Psychiatry in 2001, found a similar deficit in 63 daily marijuana smokers who hadn't smoked for up to a week. After 28 days of not smoking marijuana the effect disappeared.
Studies on brain function and mental illness cited above were conducted in adult marijuana users. How the drug affects adolescents is not completely resolved, but the data are more troubling.
A 2000 paper in the Journal of Addictive Diseases recruited 58 marijuana users and found structural changes in the brains of those who had starting smoking marijuana before age 17 but not in those who didn't start smoking until they were older.
"There's also a modest decrease in IQ if teens use heavily, though weekly users and folks who quit don't seem to show it," Earleywine says. Adolescence, he says, is a time when brain neurons are making oodles of new connections, and it's possible that a psychoactive drug such as marijuana may adversely influence that process.
Before it has any effect on the brain, marijuana smoke enters the body through the lungs. Dr. Donald Tashkin, professor of medicine at the UCLA David Geffen School of Medicine, has studied the pulmonary consequences of marijuana use for 25 years, recruiting a group of 280 heavy habitual pot smokers in the early 1980s, including some who also smoked cigarettes. (Subjects averaged three joints per day for an average of 15 years.) For comparison, he also recruited cigarette smokers who didn't use marijuana and people who didn't smoke anything.
Tashkin has done a number of studies over the decades comparing these groups. "I began with the hypothesis that regular smoking of marijuana would have an impact on the lungs qualitatively similar to the impact of regular tobacco smoking," he says. That's because the smoke of both plants are more similar than different.
Tashkin and his colleagues did find symptoms of chronic bronchitis in his marijuana-smoking group. In a 1987 study in the American Review of Respiratory Diseases, they reported that incidence of chronic cough, sputum production and wheezing was similar to that in cigarette smokers.
In a second study in the same subjects published in the American Journal of Respiratory and Critical Care Medicine in 1998, examination of the airways and the cells lining the airways found swelling, redness and increased secretions in marijuana users. Biopsies showed "extensive, widespread damage to the mucosa," Tashkin says, similar to what was seen in tobacco users. "This is amazing, because the marijuana smokers average three joints a day, but the tobacco controls smoked 22 cigarettes, suggesting that on a cigarette-to-cigarette basis, marijuana may be more damaging."