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RESEARCH WEEKLY: Marijuana and schizophrenia
By Elizabeth Sinclair Hancq
(June 14, 2023) Avid readers of this blog would know that we have written frequently on the potential connection between cannabis use, commonly known as marijuana, and severe mental illness. Past topics include medical marijuana, cannabis use’s impact on early treatment for individuals with first-episode psychosis, and the mounting evidence of a causal relationship between heavy cannabis use and the likelihood of developing schizophrenia, especially if an individual also has a genetic predisposition.
A study published last month by researchers from Denmark and the National Institute on Drug Abuse suggests young males may be especially vulnerable to the effects of cannabis on schizophrenia. In fact, assuming a causal relationship, this new research suggests that one-fifth of occurrences of schizophrenia in young males could be prevented by avoiding cannabis use disorder. This finding has huge implications for the incidence of schizophrenia as marijuana use has significantly increased among young adults and the substance progressively has become more thought of as harmless.
The study was conducted using a nationwide Danish register of seven million people aged 16-49 and included more than 45,000 individuals with schizophrenia. The researchers used this data to examine if the association between cannabis use disorder and schizophrenia varied by sex and if any differences varied by age. Similarly, the researchers examined the proportion of schizophrenia cases that are attributable to cannabis use disorder and if sex or age has any effect.
Study Results
The study authors found a stronger relationship between cannabis use disorder and schizophrenia in males as compared to females, suggesting that females with cannabis use disorder are at lower risk for developing schizophrenia than males. Additionally, for younger males, the proportion of schizophrenia cases due to cannabis use disorder is up to 30%, suggesting a significant number of cases of schizophrenia among young adult males could potentially be prevented by avoiding heavy marijuana use.
The association between cannabis use disorder and schizophrenia increased from 1972 to 2021 over the course of the study period. This finding supports previous research that has suggested that the causal relationship between heavy marijuana use and schizophrenia is in part due to the increasing potency of the drug.
While it is still important to note that cannabis use is not responsible for most of the cases of schizophrenia in Denmark, the authors write that cannabis use disorder is “a major modifiable risk factor for schizophrenia, particularly among males.”
“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 authors conclude.
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