Young men at highest risk of schizophrenia from cannabis

As the debate about drug legalisation continues in this country, yet more research showing the negative health outcomes from cannabis use, especially for young men. It is estimated that 30% of cases of schizophrenia among men aged 21-30 might have been prevented by averting cannabis use. Cannabis use disorder has consistently increased over the past five decades, and with it the cases of schizophrenia. The authors of the research note that this increase is likely linked to the higher potency of cannabis.

Young men who use cannabis frequently have an increased risk of developing schizophrenia, according to a study led by researchers at the Mental Health Services in Denmark and the National Institute on Drug Abuse (NIDA) in the United States. The study analysed detailed health records data spanning 5 decades and representing more than 6 million people in Denmark to estimate the fraction of schizophrenia cases that could be attributed to cannabis use disorder on the population level.

Researchers found strong evidence of an association between cannabis use disorder and schizophrenia among men and women, though the association was much stronger among young men. The study authors estimated that as many as 30% of cases of schizophrenia among men aged 21-30 might have been prevented by averting cannabis use.

Schizophrenia is a serious mental illness that affects how a person thinks, feels, and behaves. People with schizophrenia may seem like they have lost touch with reality, and the symptoms of schizophrenia can make it difficult to participate in usual, everyday activities.

“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,” said NIDA Director and study coauthor Nora Volkow, M.D. “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. The findings from this study are one step in that direction and can help inform decisions that health care providers may make in caring for patients, as well as decisions that individuals may make about their own cannabis use.”

Previous studies indicate that rates of daily or near daily cannabis use, cannabis use disorder, and new schizophrenia diagnoses are higher among men than women, and that early, frequent cannabis use is associated with an increased risk of developing schizophrenia.

Researchers analysed data from nationwide health registers in Denmark, which included health records data from more than 6.9 million people who were aged 16-49 at some point between 1972 and 2021. The researchers investigated how the associations between cannabis use disorder and schizophrenia varied by different sex and age groups, and how these differences changed over time.

The study team estimated that 15% of cases of schizophrenia among men aged 16-49 may have been avoided in 2021 by preventing cannabis use disorder, in contrast to 4% among women aged 16-49. For young men aged 21-30, they estimated that the proportion of preventable cases of schizophrenia related to cannabis use disorder may be as high as 30%.

This study also adds to existing evidence suggesting that the proportion of new schizophrenia cases that may be attributed to cannabis use disorder has consistently increased over the past five decades. The authors note that this increase is likely linked to the higher potency of cannabis and increasing prevalence of diagnosed cannabis use disorder over time.

“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,” said Carsten Hjorthøj, Ph.D., lead author of the study and associate professor at the Mental Health Services in the Capital Region of Denmark and at the University of Copenhagen.