Rationale: Cannabis use is rapidly increasing, while research on its mental health outcomes as they relate to psychosis, depression, and bipolar disorder is lacking.

Method: Roughly 6.7 million participants aged 16 or older in Denmark were studied over the course of 26 years. Data were collected based on CUD diagnosis from national registries including the Danish Psychiatric and National Patient Registries and the Substance Use Treatment Registries. Researchers measured psychotic and non-psychotic forms of both unipolar depression and bipolar disorder. They used the Cox proportional hazards regression to estimate hazard ratios (HRs).

Results: The main outcome was register-based diagnosis of psychotic
or nonpsychotic unipolar depression or bipolar disorder. Associations between CUD and subsequent affective disorders were estimated as hazard ratios (HRs) using Cox proportional hazards regression with time-varying information on CUD, adjusting for sex; alcohol use disorder; substance use disorder; having been born in Denmark; calendar year; parental educational level (highest attained); parental cannabis, alcohol, or substance use disorders; and parental affective disorders.

Meaning: The researchers found that CUD was associated with an increased risk of psychotic and nonpsychotic bipolar disorder and unipolar depression. These findings may inform policies regarding the legal status and control of cannabis use.