A systematic review and causation analysis found cannabis use is likely causal for schizophrenia development, with an overall odds ratio of 2.88 and double the risk for adolescent users.
Mental health professionals, policymakers on cannabis regulation, parents of adolescents, psychosis researchers.
OR 2.88 for schizophrenia risk
What the researchers found
Analysis of 18 qualifying studies (10 included in forest plot) found all reported increased risk for psychosis-like events or schizophrenia with cannabis use, with 9 of 10 reaching significance. The overall odds ratio was 2.88 (CI 2.24-3.70). Adolescent use carried roughly double the risk compared to adult use. Hill causality criteria indicated a high likelihood of causal contribution.
Why it matters
While the association between cannabis and psychosis has been extensively documented, this study goes further by systematically applying formal causation criteria. The Hill analysis suggests this is not merely correlation but likely a genuine causal contribution, particularly concerning given the twofold higher risk during adolescence when brain development is ongoing.
The numbers in context
18 studies met search criteria. 10 included in forest plot. Overall OR 2.88 (CI 2.24-3.70). Twofold higher risk for adolescent use. 9 of 10 studies showed significant increase. All 18 studies reported increased risk.
How the study worked
Systematic literature review following PRISM guidelines. Epidemiological studies and randomized clinical trials investigating cannabis-psychosis links were identified. Ten studies were included in a forest plot analysis. Confounder analysis used funnel plots. The Hill causality criteria (nine criteria for inferring causation from epidemiological evidence) were applied to estimate the likelihood of a causal relationship.
What this study cannot tell us
Funnel plot indicated potential confounder effects. Cannot rule out shared genetic vulnerability or reverse causation entirely. Included studies varied in cannabis exposure definitions and outcome measures. Most studies were observational, inherently limiting causal inference despite Hill criteria application.
How to read the evidence
Strong: systematic review with formal causation analysis, consistent findings across 18 studies, and clear dose-response and age-dependent relationships.
When this study was published
2025 study
The bigger picture
This analysis adds to growing evidence that cannabis is not just associated with but likely contributes to schizophrenia. The stronger effect during adolescence points to a window of vulnerability during brain development. As cannabis potency increases and legalization expands access, the public health implications of this causal relationship become more pressing.
Questions still open
- Is there a THC potency threshold below which schizophrenia risk is not elevated? Can the adolescent vulnerability window be more precisely defined? Would genetic screening identify individuals at highest risk from cannabis exposure?
Common questions
Does cannabis cause schizophrenia?
Is the risk higher for teenagers?
Read the original research
Does Cannabis Use Contribute to Schizophrenia? A Causation Analysis Based on Epidemiological Evidence.
Biomolecules, 15(3)
Citation
Pourebrahim, Sepehr; Ahmad, Tooba; Rottmann, Elisabeth; Schulze, Johannes; Scheller, Bertram. (2025). Does Cannabis Use Contribute to Schizophrenia? A Causation Analysis Based on Epidemiological Evidence.. Biomolecules, 15(3). https://doi.org/10.3390/biom15030368
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