Growing evidence suggests that illicit drug use may be one of the many genetic and environmental factors that trigger schizophrenia. Researchers have focused on marijuana as a possible trigger, however, they are unsure about the exact nature of the link between marijuana and psychosis. One view posits that the gene which indicates a risk for schizophrenia also predisposes someone to marijuana use. Another theory notes that people with symptoms of psychosis may use marijuana as a form of self-medication. However, more and more evidence supports the theory that marijuana itself triggers schizophrenia and other types of psychosis. Recent reviews show that people who used marijuana before 18 are more likely to develop schizophrenia. Studies also report that people who have used marijuana more than 50 times are six times more likely to be diagnosed with the disease.
Fewer studies have examined the link between schizophrenia and drugs other than marijuana. There is currently no evidence that suggests that alcohol is itself a trigger of schizophrenia, even though people with schizophrenia can relapse under the influence of alcohol. Stimulants like cocaine and methamphetamine have been connected to forms of psychoses. On the other hand, heroin users are in fact at a lower risk for psychosis. Legal and ethical standards prevent scientists from establishing a definite link between drug use and schizophrenia. To prove a connection, researchers would have to give marijuana to a person already prone to schizophrenia, which would be exposing the person to a dangerous substance. Instead, researchers must rely on longitudinal studies that detail a large group of people’s medical histories and drug use over years. The data provided by these studies take a long time to collect and depend on the participants’ memory of events, as well as their openness to discuss private information. Not all people with schizophrenia have used drugs; nor will avoiding drugs always guard against psychosis. But there is a clear risk that recreational drug use could increase the chances for a schizophrenia diagnosis.