New findings from a major Danish health study show a clear association between chronic cannabis use and higher risk of mental health disorders. The research, published in a leading medical journal, examined the medical histories of millions of Danes to understand how regular cannabis use relates to the development of depression and bipolar disorder over time. The study compared individuals who used cannabis regularly with those who did not, focusing on the transition from casual use to a disorder defined as ongoing, problematic cannabis use that disrupts health and daily life.
In total, about 56,000 participants showed signs of a cannabis use disorder. Among them, roughly 41% were diagnosed with major depressive disorder, while about 14% carried a diagnosis of bipolar disorder. The data indicate that those with cannabis use disorder faced a higher risk of mental health issues overall, including a 4.1-fold increase in the likelihood of developing a psychotic form of bipolar disorder and a 1.8-fold increase in the chance of experiencing any type of depression, compared with non-users.
Bipolar disorder is characterized by alternating episodes of low mood and periods of elevated mood, increased energy, and heightened activity. The study’s results align with earlier research that has consistently linked heavy cannabis use with depression, though the association with bipolar disorder appears less straightforward. The psychoactive component of cannabis, tetrahydrocannabinol or THC, is believed to interact with the brain’s cannabinoid system, potentially affecting information processing and behavior regulation in ways that may raise the risk of psychosis for some individuals.
Although cannabis is illegal for recreational use in Denmark, it remains accessible on the black market. The researchers emphasized the need for more work to understand how different levels and patterns of cannabis exposure influence brain function, thinking, and behavior. The findings may inform ongoing policy discussions about cannabis policy and legalization. Importantly, the study notes that stopping cannabis use does not have proven evidence for reducing the risk of developing psychiatric disorders, underscoring the need for prevention and early intervention strategies.
These insights contribute to a broader picture of how cannabis exposure relates to mental health across populations in North America and beyond. While the results do not prove that cannabis directly causes these conditions, they highlight a meaningful association that researchers in Canada and the United States are closely watching as they study dose-related effects and potential long-term outcomes for cognition and behavior. The work invites clinicians, policymakers, and communities to consider how access, education, and supportive services can mitigate risks while acknowledging the imperfect nature of current knowledge. Researchers also call for more comprehensive data to better understand who is most at risk and how to tailor prevention efforts effectively for different age groups and life circumstances. [attribution: JAMA study]