Does cannabis cause psychosis? Evidence is lacking, scientists say

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Six scientists have disputed a study published in March that argued that daily use of high-potency cannabis is “strongly linked to the risk of developing psychosis.”

The original study used data from sites treating people with psychosis in Europe and Brazil. They asserted that daily cannabis users were three times more likely to be diagnosed with their first episode of psychosis compared to people who had never used. Daily users of high-strength cannabis were five times more likely to have this diagnosis, according to the study.

READ MORE: Study links daily use of high-strength cannabis to psychosis

The study authors concluded that if high-strength cannabis wasn’t available, the incidence of psychosis would drop significantly, by specific amounts, in London and Amsterdam. 

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From the beginning, this conclusion was controversial. Attached to the original article was an opinion from a British doctor questioning the idea that if people with psychosis use cannabis at higher rates, the cannabis must have caused the psychosis.

“That’s probably the biggest issue, that correlation doesn’t equal causation,” says Kira London-Nadeau, a graduate student at the University of Montreal who heads Canadian Students for Sensible Drug Policy.

In a response published Saturday, scientists based mostly in the Netherlands raised a series of objections to the original study.

“We …  found a consistent pattern of evidence supporting a causal effect of schizophrenia risk on lifetime cannabis use. By contrast, we found little evidence for any causal effect of cannabis use on schizophrenia risk,” they wrote. In other words, people with schizophrenia may be more likely to use cannabis, but there’s no basis to think that it works the other way around.

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Both articles appeared in The Lancet, a British medical journal.

The response also criticized the March article for ignoring many other factors that could lead to the development of mental illness. London-Nadeau agrees.

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“I’m not denying that there are some cases that are catalyzed by cannabis consumption, but the point that we really want to bring forward is that this is all in conjunction with genetic predisposition, other environmental factors like poverty, childhood trauma as well,” she says. “Cannabis might be a form of self-medication, which we do see with people with psychoses.

“In terms of the overall number of psychoses that are induced by drugs, psychoses that are induced by alcohol far outnumber psychoses that are induced by cannabis, but we don’t hear about that as much.”

In the original study, high-strength cannabis was defined as anything over 10 per cent THC, which is the main psychoactive component in cannabis. Not much dried flower sold in Canada is below 10 per cent, unless it’s been bred to be high in CBD instead. It’s not hard to find dried flower that’s over 20 per cent THC.

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“You put out something that’s very simplistic and very scary and you’re implicitly proposing a very actionable solution — if cannabis weren’t around anymore, we wouldn’t have these mental-health issues in our community,” London-Nadeau argues. “The problem is that is a message that speaks to people so easily but isn’t the whole picture and isn’t the truth.”

Health Canada warns that cannabis use increases the risk of developing mental illnesses such as psychosis or schizophrenia, especially for people who start young, use frequently or have a family history of mental illness.

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