A new study confirms the robust link between cannabis use and schizophrenia among men and women but suggests that young men may be especially susceptible to schizophrenia from cannabis abuse.

Of note, investigators estimate that roughly 15% of schizophrenia cases among young males may be preventable by avoiding cannabis use disorder (CUD),

“The entanglement of substance use disorders and mental illnesses is a major public health issue, requiring urgent action and support for people who need it,” study co-author Nora Volkow, MD, director of the National Institute on Drug Abuse, says in a news release.

“As access to potent cannabis products continues to expand, it is crucial that we also expand prevention, screening, and treatment for people who may experience mental illnesses associated with cannabis use,” Volkow adds.

The study was published online May 4 in Psychological Medicine.

A Modifiable Risk Factor

The researchers analyzed Danish registry data spanning five decades and representing more than 6.9 million people in Denmark to estimate the population-level percentage of schizophrenia cases attributable to CUD.

A total of 60,563 participants were diagnosed with CUD. Three quarters of cases were in men; there were 45,327 incident cases of schizophrenia during the study period.

The overall adjusted hazard ratio (aHR) for CUD on schizophrenia was slightly higher among males than females (aHR, 2.42 vs 2.02); however, among those aged 16 to 20 years, the adjusted incidence risk ratio (aIRR) for males was more than twice that for females (aIRR, 3.84 vs 1.81).

The researchers estimate that in 2021, about 15% of schizophrenia cases among males aged 16 to 49 could have been avoided by preventing CUD, compared with 4% among females in this age range.

For young men aged 21 to 30, the proportion of preventable schizophrenia cases related to CUD may be as high as 30%, the authors report.

“Alongside the increasing evidence that CUD is a modifiable risk factor for schizophrenia, our findings underscore the importance of evidence-based strategies to regulate cannabis use and to effectively prevent, screen for, and treat CUD as well as schizophrenia,” the researchers write.

Legalization Sends the Wrong Message

In a press statement, lead investigator Carsten Hjorthøj, PhD, with the University of Copenhagen, notes that “increases in the legalization of cannabis over the past few decades have made it one of the most frequently used psychoactive substances in the world, while also decreasing the public’s perception of its harm. This study adds to our growing understanding that cannabis use is not harmless, and that risks are not fixed at one point in time.”

In a prior study, Hjorthøj and colleagues found that the proportion of new schizophrenia cases attributable to CUD has consistently increased over the past 20 years, as reported by Medscape Medical News.

“In my view, the association is most likely causative, at least to a large extent,” Hjorthøj told Medscape Medical News at the time this research was published.

“It is of course nearly impossible to use epidemiological studies to actually prove causation, but all the numbers behave exactly in the way that would be expected under the theory of causation,” Hjorthøj added.

The study received no specific funding. The authors have disclosed no relevant financial relationships.

Psychol Med. Published online May 4, 2023. Full text

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