Cannabis use showed limited brain structural changes in schizophrenia and bipolar patients, except for those who started before illness onset

MRI brain scans from 77 schizophrenia and 55 bipolar patients with cannabis history (lifetime use >10 times in one month or abuse/dependence), 97 schizophrenia and 85 bipolar patients without cannabis use, and 277 healthy controls. Cortical thickness, surface area, and subcortical volumes compared using hypothesis-driven and exploratory analyses.

This study addresses the question of whether cannabis use worsens the brain changes already present in serious mental illness. The finding that effects are limited, and largely disappear when polysubstance use is controlled for, provides reassurance while highlighting the importance of early use timing.

The distinction between cannabis use before versus after psychiatric illness onset is important for understanding risk. If cannabis primarily affects brain structure when used before illness develops, this supports the hypothesis that early cannabis exposure during critical neurodevelopmental periods may contribute to illness vulnerability rather than simply being a consequence of the disorder.

Cross-sectional design cannot determine temporal causation. Cannabis use history was retrospective and binary (user vs. non-user) without detailed dose-response data. The loss of significance after excluding polysubstance users may reflect reduced statistical power rather than absence of effect. Sample sizes within subgroups were relatively small.