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.
Depression
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.
Thinking
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.
Children
