The legalization of cannabis has been a transformative policy shift in Canada, but a recent study published in JAMA Network Open raises concerns about its potential impact on mental health, particularly schizophrenia. This extensive population-based cohort study, conducted in Ontario, analyzed data from over 13.6 million individuals aged 14 to 65 between 2006 and 2022 to assess the evolving relationship between cannabis use disorder (CUD) and schizophrenia following major policy changes, including the liberalization of medical cannabis in 2015 and the full legalization of nonmedical cannabis in 2018.
The findings reveal a stark increase in the proportion of schizophrenia cases linked to CUD. Before legalization, CUD was associated with 3.7% of schizophrenia cases, but this figure almost tripled to 10.3% in the post-legalization period. The trend was particularly alarming among young men aged 19 to 24, where CUD was linked to nearly one in five schizophrenia diagnoses (18.9%). Among females of the same age group, the figure rose from 4.8% in the prelegalization period to 12.0% after legalization.
The study also highlights a broader increase in psychotic disorders beyond schizophrenia. While the overall incidence of schizophrenia remained relatively stable throughout the study, the incidence of psychosis not otherwise specified (NOS) rose sharply—an 83.7% increase post-legalization compared to pre-legalization levels. This suggests that while cannabis policy changes may not have directly driven a surge in schizophrenia cases, they may have contributed to a rise in other severe psychiatric conditions.
A key factor in these trends is the increasing prevalence of CUD, which saw a nearly 500% increase between 2006 and 2022, from 99.2 cases per 100,000 individuals to 493.2 per 100,000. The authors suggest that the rise in high-potency cannabis products, particularly those exceeding 20% THC, could be a driving force behind the growing burden of cannabis-related mental health issues. They also point to prior research showing a dose-response relationship between cannabis use and schizophrenia risk, where higher potency and more frequent use are linked to increased risk.
One of the study’s more nuanced findings is that the increase in CUD-related schizophrenia cases was gradual rather than experiencing a sudden spike after policy changes. This suggests that factors such as changing social norms and greater cannabis availability over time played a role in the observed trends. Notably, the increase in psychosis NOS cases did accelerate following the liberalization of medical cannabis in 2015, indicating that policy shifts might contribute to broader changes in mental health diagnoses.
The implications of these findings are significant, particularly for public health policy. Young males appear to be at the highest risk, highlighting the need for targeted mental health interventions and public awareness campaigns about the potential psychiatric risks associated with cannabis use. The researchers emphasize the importance of ongoing monitoring and further investigation into the long-term consequences of cannabis policy changes, especially as the cannabis market continues to expand and diversify.
The full study, published is accessible at this link: https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2829840
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