The Proportion of Schizophrenia Cases Associated With Cannabis Use Disorder Increased in the Last Two Decades in Denmark

Epidemiologic studies have demonstrated a link between cannabis use—especially high-frequency use—and the development of schizophrenia. If this association were causal, secular increases in cannabis use and potency would lead to increasing incidence of schizophrenia. Researchers used a Danish population registry to examine trends in cannabis use disorder (CUD) and its association with development of schizophrenia to assess trends in the population-attributable risk fraction (PARF), an estimate of the proportion of schizophrenia cases that would be averted if individuals were not exposed to CUD.

• CUD increased more than 10-fold from 0.01–0.02% from 1975–1993 to nearly 0.2% in 2016.
• The adjusted hazard ratio for CUD and incident schizophrenia was nearly constant at around 4 from 1975–2016.
• PARF of CUD in schizophrenia increased from around 1–3% from 1972–1995 to 6–8% from 2010–2016.

Comments: As of 2016, 8% of schizophrenia cases in Denmark may have been attributable to cannabis exposure, a fourfold increase in two decades. These data offer further evidence of an association between cannabis use and incident schizophrenia. This association should be incorporated into public health messaging and individual risk counseling, with heightened urgency in settings with increasing availability and potency of cannabis.

Marc R. Larochelle, MD, MPH

Reference: Hjorthøj C, Posselt CM, Nordentoft M. Development over time of the population-attributable risk fraction for cannabis use disorder in schizophrenia in Denmark. JAMA Psychiatry. 2021;78(9):1013–1019.