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What’s the Real Link Between Weed and Psychosis?

A new study from the Lancet Psychiatry suggests a connection — but there are a few issues with the study

A flash light is shown on a marijuana flower called "Golden Lemons" at the "Harvest Cup" trade show, in Worcester, Mass. The event commemorates one year of legal marijuana in Massachusetts. In 2016 the state voted to legalize the possession and consumption of marijuana100 Foot Long Joint, Worcester, USA - 17 Dec 2017

A flash light is shown on a marijuana flower called "Golden Lemons" at the "Harvest Cup" trade show, in Worcester, Mass. The event commemorates one year of legal marijuana in Massachusetts. In 2016 the state voted to legalize the possession and consumption of marijuana 100 Foot Long Joint, Worcester, USA - 17 Dec 2017

Steven Senne/AP/REX/Shutterstock

As the push to legalize cannabis nationwide continues to gain ground — a record 61 percent of Americans support U.S.-wide weed legalization — those opposed to marijuana reform are doubling down, in part by citing research pointing to the health risks associated with daily cannabis consumption. The latest entry in that genre of research: a new study from The Lancet Psychiatry journal, which suggests that there may be a correlation between daily marijuana use and psychosis.

The authors of the study assessed more than 900 people in multiple European cities, all of whom had been diagnosed with at least one episode of psychosis. The researchers then asked the subjects if they had used cannabis, when they had started using it, how often they used it, and what kind of strains they smoked. Compared against a control group of 1,100 “healthy” people (i.e. people who had not experienced psychosis), the researchers found that people who smoked marijuana every day were three times more likely to have a psychotic episode than those who had never tried the drug. Additionally, that risk went up among those who had started smoking marijuana as a teenager, and it increased even further if they had smoked high-potency weed (i.e. products with more than 10 percent THC).

This is not the first time that researchers have attempted to draw a link between regular cannabis use and mental illness. In 1987, a Swedish researcher published an oft-cited study in The Lancet that found a correlation between marijuana use and schizophrenia, and as recently as last January, journalist Alex Berenson published a book about the subject, Tell Your Children: The Truth About Marijuana, Mental Illness, and Violence. (The book was criticized for cherry-picking data to support Berenson’s argument against legalization.)

But despite sensationalist coverage definitively linking marijuana use to psychosis, such research has been extremely limited, says NORML Deputy Director Paul Armentano — and The Lancet Psychiatry study is no exception. “This study is limited by an inability for authors to assess whether those most likely to express psychotic symptoms possesses a shared vulnerability or predisposition to both cannabis use and psychotic symptoms,” he told Rolling Stone, citing a previous review of marijuana and psychosis research that expresses a similar theory. In other words, it’s possible that people who are more predisposed to symptoms of psychosis may be self-medicating with marijuana, rather than the other way around.  

Additionally, the study’s subjects were based in European cities like London and Paris, where high-potency weed is widely available — but since it’s illegal and purchased through the black market, there’s no way of knowing if the cannabis contained contaminates like pesticides or mold. The subjects also self-reported the strains of the weed they smoked, which though researchers cross-checked against a European registry to determine their THC content, isn’t necessarily trustworthy. “Most… if not all of these folks are using black market cannabis, which is not analytically tested or labeled for THC potency. And the subjects are also retroactively self-reporting the quality of the cannabis consumed anyway. How do either they or the authors have any clue as to specific THC content?,” he tells Rolling Stone. 

All of this is not to say that the results of the Lancet study aren’t concerning, nor that the findings are totally without merit: a 2017 review from the National Academies of Science, Engineering, and Medicine, for instance, also found a link between marijuana and psychosis, though it was careful to note that this link was largely dependent on dosage and that the relationship “may be multidirectional and complex” — i.e., some people more inclined toward symptoms of psychosis may be more likely to smoke more marijuana, rather than the other way around. “At the end of the day we know that people suffering from psychosis generally use all intoxicants, including cigarettes, at greater levels than the general population — so it is hardly surprising that these folks also tend to use cannabis in greater percentages. But it is a leap and remains unsubstantiated to allege that cannabis exposure triggers a psychotic break in those non-predisposed,” says Armentano.

But if nothing else, the limitations of The Lancet study point to the need for more research on cannabis and its effects on long-term physical and mental health — not to mention the removal of the legal barriers that prevent such research from being conducted.

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