A study in the British medical journal The Lancet asserts that daily use of high-potency cannabis is “strongly linked to the risk of developing psychosis.”
The London-based researchers looked at data from 10 sites treating people with psychosis in Europe, and one in Brazil. About three in 10 used cannabis daily, as opposed to 6.8 per cent in a control group.
“Across the 11 sites, people who used cannabis on a daily basis were three times more likely to have a diagnosis of first episode psychosis, compared with people who had never used cannabis,” they wrote. “This increased to five times more likely for daily use of high potency cannabis.”
For the purposes of the study, high-potency cannabis was defined as over 10 per cent THC. In the Netherlands, they said that THC could get up to 67 per cent.
If it was no longer available, they argue, the incidence of psychosis in Amsterdam could drop from 37.9 to 18.8 per 100,000 people per year, and in London from 45.7 to 31.9 per 100,000 people per year.
The study was released on Tuesday.
In a published response, University of Liverpool psychologist Suzanne Gage suggested some alternative ways to look at the data.
“While cannabis use has increased in some populations, the corresponding level of psychosis incidence has not,” she wrote.
She pointed out that the relationship could work the other way: that people with psychosis could be more disposed to heavy cannabis use. Or it could be “bidirectional,” with it working in opposite ways for different people.
Gage pointed to a study that argued that some people have a genetic predisposition to both cannabis use and schizophrenia.
“The next priority is to identify which individuals are at risk from daily potent cannabis use,” she wrote.
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.