Cannabis-Linked Schizophrenia Nearly Tripled in Young Adults After Legalization

A 16-year Canadian study of 13.5 million people reveals significant increase in psychosis and schizophrenia cases among young cannabis users.

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A new study has found a surge in psychosis and schizophrenia among young adults in Canada who use cannabis.

Cannabis-linked schizophrenia cases nearly tripled from 2006 to 2022, with young men ages 19 to 24 facing the highest risk. The increase coincided with the legalization of cannabis in Ontario, Canada, in October 2018.

The new study, published in JAMA Network Open, analyzed medical records of more than 13.5 million people in Ontario. Researchers focused on the association between cannabis use disorder and the development of schizophrenia and psychosis not otherwise specified, meaning that a person has symptoms of psychosis but not enough to meet the full criteria of a psychological diagnosis.

The study found that the proportion of people showing schizophrenia symptoms while also experiencing cannabis addiction surged from 3.7 to 10.3 percent over the 16-year period.

The most significant spike, observed in young men between 19 and 24, was 18.9 percent, although a small but significant increase of 1.8 percent was also identified among women in the same age range.

Research suggests that THC, or tetrahydrocannabinol, the main psychoactive component in cannabis, may influence dopamine release and signaling in key brain regions, potentially contributing to psychotic symptoms in susceptible individuals.

These data add to growing evidence that cannabis use, especially heavy use, is associated with an increased risk of developing psychosis.

Psychosis Incidence Coincided With Medical Legalization

According to researchers, the proportion of new cases of schizophrenia linked to heavy use of high-potency cannabis occurred during a period of significant loosening of cannabis regulations in Ontario.

Despite the increase, the authors saw no link between cannabis legalization and increased schizophrenia from recreational cannabis use.

“Policy does matter,”—particularly when it comes to exposing individuals to a variety of high-potency cannabis products, Jodi M. Gilman, associate professor in psychiatry at Harvard Medical School, said in an invited commentary published in JAMA Network Open.

“As legalization of cannabis becomes more widespread, along with a rapidly expanding commercial cannabis market, a natural experiment of population exposure to commercial cannabis markets is occurring,” she stated.

Gilman noted that these results point to the intricate nature of cannabis legalization’s effect on mental health.

“These findings ... highlight the methodical challenges of linking the biological causality of a complex psychiatric illness, such as schizophrenia, with a cannabis policy change,” Gilman wrote.

The researchers who conducted the study stated that there are three key challenges in linking mental health outcomes with drug policy changes.

The first is that it can take years to observe changes in the population after cannabis legalization. There are also challenges with establishing a clear timeline between cannabis use and psychotic symptoms. The potency and type of cannabis products used is also usually unclear, creating methodological challenges, the researchers stated.