Cannabis-induced psychosis had the highest relapse rate and shortest time between episodes

What the researchers found

Cannabis-induced psychosis carried a 2.42x higher risk of relapse compared to alcohol-induced psychosis. Overall, 20% of people with any substance-induced psychosis relapsed within 2 years, with a median time to relapse of 126 days.

Why it matters

Substance-induced psychosis is often treated as a one-time event, but this study shows that one in five people relapse within two years, with cannabis-related cases at particularly high risk. This has implications for follow-up care planning.

The numbers in context

7,320 people studied. 20% (n=1,463) relapsed. Cannabis-induced: 25.7% relapse, HR 2.42. Multi-substance: 23.8% relapse, HR 1.81. Methamphetamine: 19.7% relapse, HR 1.49. 83.3% relapsed with the same substance type. Median relapse: 126 days.

How the study worked

Population-based register study linking Swedish nationwide data for 7,320 people with first-time substance-induced psychosis between 2006 and 2016, with 2-year follow-up using multivariable Cox models.

What this study cannot tell us

Register-based study cannot capture psychotic episodes not leading to hospitalization. Cannabis potency, frequency of use, and other contextual factors were not available in the registers.

How to read the evidence

Large population-based register study covering an entire country over a decade, with multivariable adjustment for confounders. Register data provides comprehensive capture of hospitalizations.

When this study was published

Published in 2025, data 2006-2016.

The bigger picture

As cannabis potency and availability increase globally, understanding the longitudinal trajectory of cannabis-induced psychosis becomes crucial for mental health services planning and resource allocation.

Questions still open

• What interventions after a first cannabis-induced psychosis episode could reduce the 25.7% relapse rate? Does cannabis potency influence relapse risk?

Common questions

Read the original research

Relapse in substance-induced psychosis and associated risk factors. A Nationwide register-linkage study from Sweden.

Addiction (Abingdon, England), 120(7), 1422-1430

Citation

Ellilä, Venla; Taipale, Heidi; Tiihonen, Jari; Mittendorfer-Rutz, Ellenor; Niemelä, Solja. (2025). Relapse in substance-induced psychosis and associated risk factors. A Nationwide register-linkage study from Sweden.. Addiction (Abingdon, England), 120(7), 1422-1430. https://doi.org/10.1111/add.70014

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