“Our study shows that daily cannabis use, especially of high potency cannabis, is strongly linked to the risk of developing psychosis,” lead author Marta Di Forti of King’s College London wrote in an email sent to Popular Science. The researchers define this as the risk of developing a psychotic disorder (it’s a technical term) such as schizophrenia and having symptoms such as hearing voices and experiencing delusions or paranoia. They studied 901 patients who experienced their first episode of psychosis and a further 1237 patients who had never had such an episode. The subjects came from 11 different sites across Europe and Brazil. They found that daily cannabis use and the use of highly potent strains were both correlated with a higher rate of psychosis. It’s long been known that there can be a causal link between marijuana use and the onset of such symptoms. This new study “focuses on identifying the pattern of use that carries the highest risk and how this affects rates of psychotic disorder,” Di Forti writes. “Given the increasing availability of high-potency cannabis, this has important implications for public health,” the King’s College researchers wrote in the paper. But in looking at the link between marijuana and psychosis, “I think sometimes it’s useful to zoom out a little bit,” says Ian Hamilton, a University of York mental health and addiction scientist. “We’ve known about this association for a long time,” he says, which leads him to question what this new research adds to the picture. At the population scale–that is, the scale that most public health officials think about–the risk of psychosis as a result of marijuana consumption is just one in 20,000, Hamilton says. There are many more worrying behaviors related to recreational marijuana use, particularly among young people, he says. “I think if money was no issue, if there were plenty of research money, I’d welcome studies like this,” Hamilton says. But “… money for research, particularly on drug use and mental health, is quite hard to come by.” In the UK, where he works, a much more widespread marijuana-related public health concern is the number of young people who mix tobacco and marijuana in their joints and become addicted to nicotine as a result. Cigarette smoking is a leading preventable cause of death in the United Kingdom, causing nearly one in five preventable deaths, according to Cancer Research UK. Rather than trying to add more levels of detail into the picture of how cannabis use is linked to psychosis, he says, “I think what would be far more interesting [and] far more useful is to try and work out who has a problem before they’re exposed to cannabis.” That way, people at high risk of developing psychosis could be identified and reached by targeted public health interventions Hamilton is concerned that general public health messaging about psychosis being a risk of marijuana smoking misses the mark. “I think we have to be careful that we don’t exaggerate the risk,” he says. While recreational marijuana use isn’t the cause of “reefer madness,” there are other reasons to keep an eye on your marijuana consumption. For one thing, smoking it is linked to a variety of ailments (smoke is just bad for your lungs, in general). Compared to factors like these, a 1:20,000 risk of developing symptoms of psychosis is pretty negligible, and it’s unlikely to stop many people from consuming marijuana, says Hamilton. “Although psychosis is devastating, and you wouldn’t wish it on anyone, the risk of developing psychosis is very very small compared to the number of people who are exposed to [marijuana],” Hamilton says. Non-targeted public health warnings about it can make authorities (the Surgeon General, for instance, who has never issued a public health warning about marijuana smoking) seem less credible, especially to young recreational users of marijuana, he says. If they hear about the risk of psychosis, but then nobody they know is one of the 1 in 20,000 marijuana users who develop it, they may stop listening to other public health messaging about the potential harms of marijuana smoking.
