Cannabis Likely Contributes to Schizophrenia Development, Especially in Adolescents

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.

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.

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.

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.

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.