Many studies have linked marijuana use with early onset of psychosis. The question is, does smoking marijuana cause earlier psychosis? A new review of 83 studies involving more than 22,000 participants seeks an answer.
The meta-analysis found that people who smoked marijuana developed psychotic disorders an average 2.7 years earlier than people who did not use cannabis. But the review also found that people who used any illegal drug suffered psychosis two years earlier than non-users, not a large difference.
While alcohol use was not associated with early onset, the studies reviewed could not rule out the influence of cigarette smoking, which is a common habit of people with psychotic disorders and those who smoke marijuana. In many of the countries from which the data was gathered, in fact, cannabis is typically smoked mixed with tobacco. The researchers argue that cigarette smoking—unlike marijuana— does not worsen hallucinations or paranoia in patients with schizophrenia, so they believe that tobacco does not account for earlier onset.
Led by Mathew Large of the University of New South Wales in Australia, the authors of the new research review also focused on other factors that may have contributed to the association between pot smoking and early psychosis in previous studies. For instance, it’s known that men are both more likely to take drugs and to develop psychosis at an earlier age than women, and the preponderance of men in past studies might have contributed to the connections found. It’s also known that young people, including those who have schizophrenia, are more likely to smoke pot than older folks, so previous studies that put upper age limits on their participants could have overstated the marijuana-psychosis link.
But based on the meta-analysis, researchers found that the link persisted independently of these factors. In the paper, published in the Archives of General Psychiatry, they conclude that their results “provide evidence for a relationship between cannabis use and earlier onset of psychotic illness and they support the hypothesis that cannabis use plays a causal role in the development of psychosis in some patients.”
The research adds to the evidence that marijuana smoking may be dangerous for people with a family history of psychosis. TIME recently reported on a study that found that among 190 patients with schizophrenia, 121 of whom had used marijuana, cannabis appeared to affect the age of psychosis onset in a subgroup of 44 patients. The affected patients either had their first symptoms within a month of smoking pot for the first time, or experienced a severe worsening of psychotic symptoms each time they smoked.
The cannabis-sensitive patients not only developed psychosis nearly three years earlier than patients who were unaffected by the drug, but they also had three times the number of close relatives with psychotic disorders.
In the new review, researchers also cite studies linking a particular gene to sensitivity to cannabis, which could help explain why most marijuana smokers don’t have a higher risk of schizophrenia, and why even many people with schizophrenia do not have earlier onset connected with cannabis use.
Still, none of the data linking marijuana use and psychosis can prove causality or sufficiently explain why rates of schizophrenia have remained stable or even declined since the 1950s, while marijuana use has increased exponentially. Unlike rates of cigarette smoking and lung cancer, which rise in tandem, marijuana smoking rates in the population do not correlate with higher rates of schizophrenia.
Either way, it’s now clear that if you have any close relatives with schizophrenia or other psychotic disorders, it’s better to steer clear of marijuana, particularly during adolescence when the brain is undergoing significant development.