Within-person analysis suggests cannabis causally impacts psychosis risk, but not the reverse
Using a within-person design that removes confounding from fixed characteristics, prior cannabis use was associated with a sevenfold increase in odds of psychotic experiences, while prior psychotic experiences did not predict cannabis use.
Quick Facts
What This Study Found
In the fixed-effects model, prior cannabis use predicted subsequent psychotic experiences (adjusted OR = 7.03, 95% CI: 2.39-20.69), but prior psychotic experiences did not predict subsequent cannabis use (adjusted OR = 0.59, 95% CI: 0.21-1.71).
Key Numbers
Combined cohort: 7,998 at baseline. Follow-up: approximately 10 years across 4 time points. Cannabis to psychosis OR: 7.03 (95% CI: 2.39-20.69). Psychosis to cannabis OR: 0.59 (95% CI: 0.21-1.71).
How They Did This
Combined two general population cohorts (n = 7,998 at baseline) with repeated interviews over approximately 10 years (4 time points). Used fixed-effects within-person models where each individual serves as their own control, removing confounding by any stable characteristic. Adjusted for time-varying confounders.
Why This Research Matters
Most cannabis-psychosis research uses between-person comparisons, which cannot fully control for genetic and environmental confounders. This within-person design substantially strengthens the causal inference that cannabis impacts psychosis.
The Bigger Picture
The debate about whether cannabis causes psychosis or whether people predisposed to psychosis seek out cannabis has persisted for decades. This within-person approach provides some of the strongest evidence yet for a directional effect from cannabis to psychosis.
What This Study Doesn't Tell Us
Self-reported measures of both cannabis use and attenuated psychotic experiences. Attenuated psychotic experiences are not the same as clinical psychosis diagnoses. Time-varying confounders may not be fully captured.
Questions This Raises
- ?Does the strength of this causal relationship vary by cannabis potency or frequency of use?
- ?At what threshold of use does the risk become meaningful?
Trust & Context
- Key Stat:
- 7x increased odds of psychotic experiences after cannabis use
- Evidence Grade:
- Within-person design across two population cohorts with 10-year follow-up provides strong causal inference, though based on self-report.
- Study Age:
- Published in 2021.
- Original Title:
- Schizophrenia and the Environment: Within-Person Analyses May be Required to Yield Evidence of Unconfounded and Causal Association-The Example of Cannabis and Psychosis.
- Published In:
- Schizophrenia bulletin, 47(3), 594-603 (2021)
- Authors:
- van Os, Jim(19), Pries, Lotta-Katrin(2), Ten Have, Margreet(2), de Graaf, Ron, van Dorsselaer, Saskia, Bak, Maarten, Wittchen, Hans-Ulrich, Rutten, Bart P F, Guloksuz, Sinan
- Database ID:
- RTHC-03589
Evidence Hierarchy
Follows a group of people over time to track how outcomes develop.
What do these levels mean? →Frequently Asked Questions
What makes within-person analysis stronger than typical studies?
Each person serves as their own control, automatically removing confounding from all stable characteristics like genetics, childhood environment, and personality traits.
Does this prove cannabis causes psychosis?
It provides strong evidence for a causal direction from cannabis to psychotic experiences, but the study measured attenuated psychotic experiences in the general population, not clinical psychosis diagnoses.
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Cite This Study
https://rethinkthc.com/research/RTHC-03589APA
van Os, Jim; Pries, Lotta-Katrin; Ten Have, Margreet; de Graaf, Ron; van Dorsselaer, Saskia; Bak, Maarten; Wittchen, Hans-Ulrich; Rutten, Bart P F; Guloksuz, Sinan. (2021). Schizophrenia and the Environment: Within-Person Analyses May be Required to Yield Evidence of Unconfounded and Causal Association-The Example of Cannabis and Psychosis.. Schizophrenia bulletin, 47(3), 594-603. https://doi.org/10.1093/schbul/sbab019
MLA
van Os, Jim, et al. "Schizophrenia and the Environment: Within-Person Analyses May be Required to Yield Evidence of Unconfounded and Causal Association-The Example of Cannabis and Psychosis.." Schizophrenia bulletin, 2021. https://doi.org/10.1093/schbul/sbab019
RethinkTHC
RethinkTHC Research Database. "Schizophrenia and the Environment: Within-Person Analyses Ma..." RTHC-03589. Retrieved from https://rethinkthc.com/research/van-2021-schizophrenia-and-the-environment
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.