Cannabis may increase schizophrenia risk, especially among young men: study

A man smokes a marijuana cigarette as he takes part in a march marking World Cannabis Day, officially observed annually on April 20th, to call for the full legalization of the drug in Quito, Ecuador, Thursday, May 5, 2022.

Heavy cannabis use appears to increase the risk of schizophrenia, especially among young men, warns a new Danish epidemiological study.

Researchers at the University of Copenhagen examined the medical records of nearly seven million people. They concluded that some 3,000 diagnoses of schizophrenia, or about one-third of the total number of diagnoses, could have been prevented if men aged 21 to 30 had not developed problematic cannabis use.

The researchers also note that the quality of marijuana available in Denmark and the number of schizophrenia diagnoses increased simultaneously between 2006 and 2016.

"The first studies that suggested there was a link between cannabis use, especially in adolescence, and the development of psychosis in adulthood must be about 20 years old now, and the data that point in that direction have continued to accumulate," noted Dr. Marc-André Roy, who is a full professor of psychiatry and neuroscience in the Faculty of Medicine at Laval University.

The interaction between genes and the environment can be complex, he said. But today, researchers are increasingly convinced that excessive cannabis use increases the risk of schizophrenia, not that individuals with a genetic predisposition to schizophrenia become heavy cannabis users.

Assuming a causal link, the researchers wrote, by 2021, 15 per cent of schizophrenia cases would have been prevented in men who didn't abuse cannabis and 4 per cent in women.

Among young men, the proportion of schizophrenia cases associated with cannabis abuse that could be prevented could rise to 25 per cent or even 30 per cent, they added.

Men are known to be heavier cannabis users than women, but that alone probably doesn't explain the gender gap, Roy said.

"There are many aspects of neurodevelopment that are influenced by gender," he said. "There are several phenomena that are powerfully modulated by sex hormones, and that's kind of always what we think about. But in this study, it goes beyond the fact that boys use more than girls. With equal use, cannabis has a greater effect in men."

For example, they can't rule out that women's sex hormones protect them from the harmful effects of cannabis. They are less likely than men to experience psychosis, Roy said, and when they do, it usually occurs later in life.

Still assuming a causal link, the Danish researchers write, the results "suggest that males compared to females of the same age may be more susceptible to the psychotogenic effects of cannabis on schizophrenia." However, they say, more research is needed to better understand the mechanisms responsible for this greater vulnerability in young men.

At a population level, the study authors say, problematic cannabis use appears to represent a significant modifiable risk factor for schizophrenia, particularly for men.

"We know that in populations where cannabis with a higher concentration is used, there seems to be more cannabis-related psychosis," said Roy. "Looking back, it's starting to meet all the criteria pretty well. It's starting to be hard to say it has nothing to do with it."

The findings of the study were published by the medical journal Psychological Medicine.

This report was first published in French by The Canadian Press on May 11, 2023.