Cannabis addiction appears to have ‘serious’ long-term effect in young men

‘Study adds to growing understanding that cannabis use is not harmless’

Young men who showed signs of cannabis addiction had an increased risk of developing schizophrenia, a new study warned.

Researchers, including those from the US National Institutes of Health, analysed health records data spanning decades and representing more than six million people in Denmark to estimate the fraction of schizophrenia cases that could be attributed to cannabis use disorder.

Schizophrenia is a serious mental disorder that affects how a person thinks, feels and behaves, with patients seeming to have lost touch with reality, making everyday activities almost impossible to complete in some cases.

People with cannabis use disorder are unable to stop using the drug despite its negative consequences in their lives, which also makes everyday activities difficult.

The new study, published in Psychological Medicine, provides strong evidence of a link between cannabis use disorder and schizophrenia among men and women, with a much stronger association among young men.

Scientists estimate that as many as a third of cases of schizophrenia among men aged 21-30 might have been prevented by averting cannabis use disorder.

“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 coauthor Nora Volkow, the director of the National Institute on Drug Abuse (NIDA) at the NIH, said.

“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,” Dr Volkow said.

In the research, scientists investigated how the associations between cannabis use disorder and schizophrenia varied by different sex and age groups, and how these differences changed over time.

They sought to estimate the proportion of all schizophrenia cases that may be attributed to cannabis use disorder specifically, across sex and age groups.

The study found that about 15 per cent of cases of schizophrenia among men aged 16-49 may have been avoided in 2021 by preventing cannabis use disorder.

But for young men aged 21-30, researchers estimated that the proportion of preventable cases of schizophrenia related to cannabis use disorder may be as high as 30 per cent.

Scientists also warn that the proportion of new schizophrenia cases that may be attributed to cannabis use disorder has consistently increased over the past five decades.

This increase they say is likely linked to the higher potency of cannabis and increasing prevalence of diagnosed cannabis use disorder over time.

“This study adds to our growing understanding that cannabis use is not harmless, and that risks are not fixed at one point in time,” Carsten Hjorthoj, lead author of the study from the University of Copenhagen said.

“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,” Dr Hjorthoj said.

Researchers call for further studies to examine the mechanisms underlying the higher vulnerability of young men to the effects of cannabis on schizophrenia.