A new study suggests that individuals who are addicted to marijuana have an increased risk of depression and bipolar disorder. The study, which was published in JAMA Psychiatry, was conducted by researchers at Aarhus University Hospital in Denmark. The team assessed the potential link between cannabis use disorder and increased prevalence of bipolar and unipolar depression – both psychotic and nonpsychotic subtypes.
The longitudinal cohort study used national registrant data for all individuals born in Denmark before 2006 who were ≥16 years old and living in the country between 1995 and 2021. The analysis included 6.65 million individuals who were followed up for more than 119.53 million person-years. Overall, just 60,696 individuals (0.9%) had received a cannabis use disorder diagnosis during the follow-up period, and another 260,746 (3.9%) had developed an affective disorder relevant to the analysis.
The study found that individuals with a cannabis addiction have a nearly two-fold increased risk of depression, and nearly three-fold increased risk of bipolar disorder. While the risk of bipolar disorder was increased in both men and women, the risk was more pronounced in men. Individuals who used cannabis had a more than four-fold increased risk of psychotic bipolar disorder.
The researchers identified several limitations in their study, including an uncertainty of classified cannabis use disorder diagnoses and affective disorders, as well as the risk of detection bias. However, they stressed that their findings are possibly the largest investigation to date into associations between cannabis use disorder and affective disorders.
The team suggested that their data implicate the legalization and control strategies surrounding cannabis. They called for improved knowledge on the dose-dependent effects of cannabis use on brain, cognition, and behavior. Finally, they emphasized the need for identification of risk factors for transition from cannabis use (disorder) to psychiatric disorders, and the effects of cannabis cessation on long-term psychiatric risk.
The findings of this study have important implications for the standards for cannabis legalization and regulation, as well as efforts to increase public knowledge regarding cannabis’ impact on mental health and cognition. With the increased use of cannabis in many countries and territories, it is essential to continue to research the long-term effects of cannabis use, particularly in relation to mental health. This study adds to the growing body of evidence suggesting that cannabis use disorder may increase the risk of developing psychiatric disorders, and highlights the need for increased awareness and education on the potential risks of cannabis use.