Early cannabis use linked to drug-induced psychosis while later use linked to schizophrenia in homeless adults

Early cannabis exposure (by age 15) was associated with increased risk of substance-induced psychosis (OR=1.09, p<0.05). Later first use (after age 15) increased risk of schizophrenia or schizoaffective disorder (OR=2.19, p<0.01). No cognitive differences were found between groups, though brain imaging showed differences in the lateral orbitofrontal cortex and white matter tract diffusivity.

Cross-sectional study of 437 homeless/precariously housed adults recruited from single-room occupancy hotels in Vancouver's Downtown Eastside. Psychiatric diagnoses determined clinically. Psychotic symptoms measured with 5-factor PANSS. Neurocognitive battery and structural/diffusion tensor MRI completed.

The differential association between timing of cannabis use and type of psychotic disorder suggests different developmental mechanisms. This has implications for understanding how cannabis affects the developing brain at different stages.

The finding that timing of first cannabis use predicts different psychotic outcomes challenges the idea of a single "cannabis-psychosis pathway" and suggests that the developing brain may be vulnerable to different types of psychotic disruption at different ages.

Cross-sectional design cannot establish causation. Retrospective recall of age of first use. Homeless/precariously housed population has many confounding risk factors (poverty, trauma, polysubstance use). Selection bias inherent in studying this population.