Reefer madness — the idea that marijuana makes you crazy — isn’t new. Long ago turned into a vintage joke, this idea has been mostly relegated to history books and fridge magnets — until now. But it turns out the vintage joke is still relevant.

People who used high-potency cannabis with more than 10 percent THC levels on a daily basis were almost five times more likely to have a psychotic episode, compared to those who didn’t, according to a new study in The Lancet Psychiatry.

The study was conducted by researchers at King’s College London between 2010 and 2015. They collected data from 901 cannabis users in Europe and Brazil who had been diagnosed with first-episode psychosis. The control group included 1,237 people from the general population.

Researchers concluded that if high-potency THC were not available, the number of first-episode psychosis would have been 30 percent less in London and 50 percent less in Amsterdam.

Listen to GHOGH with Jamarlin Martin | Episode 46: Kai Bond
Jamarlin talks to Kai Bond, managing partner at Comcast Ventures Catalyst Fund. They discuss the Fyre Festival being flagged during due diligence and Kai’s observation that most African-American entrepreneurs are under-negotiating. They also discuss a Washington Post article suggesting Facebook is psychopathic.

“One should be careful though not to infer causality which the study could and did not do,” Reid K Hester wrote in Psychology Today. ” Other things like history of childhood trauma could have impacted the findings, Hester wrote.

The study also found that using less potent strains of cannabis daily increased the risk of psychosis three-fold.

Officials at the marijuana advocacy group NORML said the study is premature and potentially flawed.

“Cannabis use has increased over the years, which puts doubt on the study as incidents of psychosis have generally stayed the same,” said Karen Goldstein, director of NORML Florida, according to the Miami Herald.

Cannabis has been mostly illegal until recently, so there have been few studies on the full range of its effects, positive or negative. 

High-THC cannabis products are more common in the market now, so it’s a concern, said Krista M. Lisdahl, a clinical neuropsychologist at the University of Wisconsin, Milwaukee, in an NPR interview.

“This is more evidence that the link between cannabis and psychosis matters,” Lisdahl said.