Connection Between Heavy Marijuana Use & Psychosis Explored

By Kelly Burch 03/22/19

Those who used high potency marijuana—with a THC content above 10%—quadrupled their risk for psychosis, according to a new study. 

Two people sharing a marijuana joint

Legal weed is becoming more socially acceptable, but a study published this week reaffirms the connection between heavy marijuana use and psychosis—highlighting the fact that despite its legal status, the drug continues to have real effects on health. 

The study, published in The Lancet Psychiatry, looked at the connection between marijuana use and first-time psychotic episodes. The authors found that people who smoked weed frequently and who used high-potency cannabis were more likely to experience psychotic episodes than people who did not use marijuana. 

Krista M. Lisdahl, who works at the University of Wisconsin as a clinical neuropsychologist, said that the study contributes to the body of evidence that indicates a link between marijuana use and psychosis. 

"This is more evidence that the link between cannabis and psychosis matters,” Lisdahl, who was not involved with the study, told NPR

The study authors found that people who used pot daily were three times more likely to have a psychotic episode than people who didn’t use marijuana. Those who used high-potency marijuana—with a THC content above 10%—quadrupled their risk for psychosis. The increased risk was even more pronounced for people who starting using cannabis before they were 15. 

Marta Di Forti, the study author and a psychiatrist, said that the findings are concerning because high-potency marijuana is more readily available than ever. "Almost 20 years ago, there wasn't much high-potency cannabis available [in the market],” she said. 

The cities that had the most availability of high-potency weed—London, Paris and Amsterdam—also had the highest rates of new psychosis cases. This finding strengthened the connection between THC content and psychosis, says Suzanne Gage, a psychologist and epidemiologist who wrote a commentary that accompanied the study. 

"That's a really interesting finding, and that's not something anyone has done before," she said. 

Lisdahl agreed. "One of the things that's really novel is that they could show that variation of use and potency of cannabis was related to rates of first-episode psychosis,”she said. 

Despite the link, Dr. Diana Martinez, who researches addiction at Columbia University, said that the research does not point to a causal relationship between marijuana use and psychosis. 

"You can't say that cannabis causes psychosis," she said. "It's simply not supported by the data.”

Instead, there are likely multiple factors that influence the emergence of psychosis.

"In all psychotic disorders, there is this multiple hit hypothesis,” she said. Those factors include genetic and environmental causes, possibly including marijuana exposure. 

Despite this, Di Forti says that the study should be cause for caution in people who use marijuana regularly. 

"There are people across the world who use cannabis for a variety of reasons," she said. "Some of them recreationally, some of them for medicinal purposes. They need to know what to look for and ask for help, if they come across characteristics of a psychotic disorder.”

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Kelly Burch writes about addiction and mental health issues, particularly as they affect families. Follow her on TwitterFacebook, and LinkedIn.