Since the days of Reefer Madness, scientists have sought to understand the complex connection between marijuana and psychosis. Cannabis can cause short-term psychotic experiences, such as hallucinations and paranoia, even in healthy people, but researchers have also long noted a link between marijuana use and the chronic psychotic disorder, schizophrenia.
Repeatedly, studies have found that people with schizophrenia are about twice as likely to smoke pot as those who are unaffected. Conversely, data suggest that those who smoke cannabis are twice as likely to develop schizophrenia as nonsmokers. One widely publicized 2007 review of the research even concluded that trying marijuana just once was associated with a 40% increase in risk of schizophrenia and other psychotic disorders.
But here’s the conundrum: while marijuana went from being a secret shared by a small community of hepcats and beatniks in the 1940s and ’50s to a rite of passage for some 70% of youth by the turn of the century, rates of schizophrenia in the U.S. have remained flat, or possibly declined. For as long as it has been tracked, schizophrenia has been found to affect about 1% of the population.
One explanation may be that the two factors are coincidental, not causal: perhaps people who have a genetic susceptibility to schizophrenia also happen to especially enjoy marijuana. Still, some studies suggest that smoking pot can actually trigger the disease earlier in individuals who are predisposed, and yet researchers still aren’t seeing increases in the overall schizophrenia rate or decreases in the average age of onset.
In recent months, new research has explored some of these issues. One study led by Dr. Serge Sevy, an associate professor of psychiatry at the Albert Einstein College of Medicine in New York City, looked at 100 patients between the ages of 16 and 40 with schizophrenia, half of whom smoked marijuana. Sevy and colleagues found that among the marijuana users, 75% had begun smoking before the onset of schizophrenia and that their disease appeared about two years earlier than in those who did not use the drug. But when the researchers controlled for other factors known to influence schizophrenia risk, including gender, education and socioeconomic status, the association between disease onset and marijuana disappeared.
