Cannabis-Related ER Visits Carried the Highest Risk of Later Developing Schizophrenia in a Study of Nearly 10 Million People

Among 9.8 million people without prior psychosis, those with ER visits for substance-induced psychosis had a 163-fold increased risk of transitioning to schizophrenia (3-year risk: 18.5% vs 0.1%). Cannabis-induced psychosis had the highest transition risk (aHR 241.6). Even substance use ER visits without psychosis carried a 9.8-fold risk, and because they were far more common, they accounted for 3 times more total transitions than psychosis visits.

Population-based retrospective cohort study of 9.8 million individuals aged 14-65 in Ontario, Canada (January 2008 to March 2022), with no prior psychotic disorder. Used cause-specific hazard models to estimate transition risk to schizophrenia spectrum disorder.

This is one of the largest studies to quantify the pathway from substance-related ER visits to schizophrenia. The finding that cannabis carried the highest transition risk among all substances, and that even non-psychotic substance use visits predicted elevated risk, has major implications for early intervention.

The study highlights a critical but often overlooked finding: while substance-induced psychosis gets the most attention, the sheer volume of non-psychotic substance use ER visits means they produce far more schizophrenia cases overall. This suggests screening and follow-up for all substance-related ER visits, not just psychosis episodes.

Observational design cannot prove causation. Relies on ER diagnostic coding which may miss or misclassify cases. Ontario has universal healthcare which may limit generalizability to other systems. Cannot account for frequency or potency of substance use.