Young people with a parent or sibling affected by psychosis have a roughly one in 10 chance of developing the condition themselves—even if they never smoke pot. Regular marijuana use, however, doubles their risk—to a one in five chance of becoming psychotic.
In comparison, youths in families unaffected by psychosis have a 7 in 1,000 chance of developing it. If they smoke pot regularly, the risk doubles, to 14 in 1,000.
For years, now, experts have been sounding the alarm about a possible link between marijuana use and psychosis. One of the best-known studies followed nearly 50,000 young Swedish soldiers for 15 years. Those who had smoked marijuana at least once were more than twice as likely to develop schizophrenia as those who had never smoked pot. The heaviest users (who said they used marijuana more than 50 times) were six times as likely to develop schizophrenia as the nonsmokers.
So far, this research shows only an association between smoking pot and developing psychosis or schizophrenia later on. That’s not the same thing as saying that marijuana causes psychosis.