The results are published online in the Archives of General Psychiatry.
The researchers analyzed 83 studies comprising 8,167 people with a psychotic illness who used marijuana or other psychoactive substances and 14,352 with a psychotic illness who did not. Those who used marijuana were close to three years younger when they developed a psychotic illness such as schizophrenia compared with those who did not use marijuana.
People who used any type of illicit substances (including, but not limited to marijuana), were two years younger when they were diagnosed with a psychotic illness than their drug-free peers. Alcohol use had no bearing on the age of onset of psychotic illness, the study shows.
“It is increasingly clear that marijuana is a cause of schizophrenia, and that the schizophrenia caused by cannabis starts earlier than schizophrenia with other causes,” study researcher Matthew Large of Prince of Wales Hospital in New South Wales, Australia, says in an email. “Young people are at particular risk.”
Marijuana’s Role in Psychotic Illness
Exactly how marijuana use affects risk of psychotic illness is not fully understood, but “marijuana, when smoked, produces hundreds of chemicals, over 50 are psychoactive,” he says.
“There is not so much evidence for the widely held view those patients self-medicate with marijuana,” he says. “Marijuana smoking almost always comes before psychosis and few patients with psychosis start smoking [marijuana] for the first time.”
More than 80% of the patients in the study had schizophrenia, but there were some other forms of psychosis identified among marijuana users. “The picture looked similar irrespective of the type of psychosis,” Large says.
Christoph U. Correll, MD, medical director of the Recognition and Prevention Program at the Zucker Hillside Hospital in Glen Oaks, N.Y., says that “marijuana usage can contribute to psychiatric disorders, but many people use it and don’t have a psychiatric disorder.”
More than 16 million people in the U.S. use marijuana on a regular basis, and many of them began using in their teenage years, according to information cited in the new report.
Perhaps marijuana use is one of the factors that contribute to the development of a psychotic illness among people who are genetically predisposed to such illness, Correll says.
When asked if he thought the marijuana was the chicken or the egg, he says: “I think it is a mixture. Some people affected by illness may choose pot to cope with symptoms, but at least for a subgroup, use of pot at an earlier age may hasten the onset of psychotic illness.”
The message is clear, he says.
“Try to use illicit drugs as little as possible,” he says. “There is a chance that you may have a genetic predisposition for a psychiatric disorder and this can lead to an earlier outbreak or conversion to psychosis than would have happened otherwise.”