This week, a published study announced that potent cannabis significantly increases the risk of psychosis. So much so, found the authors of The Lancet paper, that around 30% of first-episode psychosis experiences in London and 50% of those in Amsterdam could be prevented if high-potency cannabis were not available in those cities.
Those findings are of course concerning, and they’re part of a body of work showing the link between marijuana and poor mental health: Earlier research found cannabis is associated with psychotic symptoms, such as paranoia; scientists issued a public health warning about the drug in 2016; and a recently published book on marijuana, Tell Your Children, highlights a 2013 paper that found a link between weed and violence, as well as increased rates in violence in the state of Washington shortly after it legalized weed in 2012.
Ultimately, though, they’re far from conclusive. One major question is whether cannabis causes psychosis, or if those who are predisposed to psychosis are also more likely to seek out marijuana. So far, evidence points in both directions. Some research suggests cannabis both increases the likelihood of schizophrenia, and that those with the mental condition tend to use marijuana more than the general population. Meanwhile, a 2017 study showed that there’s a genetic overlap between tendency for cannabis use and schizophrenia.
It’s also important to note that the Lancet paper focused on daily users of high-potency cannabis, rather than those who only smoke occasionally; as might be expected, some of the clearest evidence of risk, such as the recent Lancet paper, focuses on heavy users. Furthermore, the authors point out that many daily users did not develop a psychotic disorder; their findings on the percentage of psychosis cases that could be prevented is based on statistical analysis, but further work is necessary to identify the other relevant factors.
Since marijuana remains illegal in much of the world, there are major regulation and restrictions around researching it. So scientists really don’t yet know as much as they should. There’s also evidence that the drug causes benefits: there’s considerable evidence that it can treat chronic pain and insomnia, for example, and the US Food and Drug Administration has approved a cannabinoid to treat a form of epilepsy. Incredibly, Harvard scientists recently found that smoking weed is associated with a higher sperm count, though further research is needed to determine exactly how cannabis affect fertility. Overall, few drugs come without risks and there is certainly considerable evidence that cannabis is associated with psychosis. But marijuana’s reputation in North America and Western Europe has veered between the hippy conception of the drug a natural risk-free medicine, and 1970s conservative politicians’ view that weed is a highly dangerous substance that causes criminality. The truth is somewhere in between.