Major London study: one-quarter of new psychosis cases attributed to high-potency cannabis
The landmark GAP study of 410 first-episode psychosis patients found that one-quarter of new cases could be attributed to high-potency cannabis use, with genetic and environmental factors acting together.
Quick Facts
What This Study Found
In 410 first-episode psychosis patients and 370 controls in South London, approximately 25% of new psychosis cases were attributable to high-potency cannabis use. Childhood adversity (parental loss, abuse, bullying), ethnic minority status, and cannabis use were major environmental risk factors, operating on a background of polygenic genetic risk. Continued high-potency cannabis use also predicted poorer long-term outcomes.
Key Numbers
410 FEP patients, 370 controls; 25% of new psychosis cases attributable to high-potency cannabis; childhood adversity, ethnic minority status, and cannabis as key environmental risks.
How They Did This
Prospective multidisciplinary case-control study (GAP Study) recruiting 410 first-episode psychosis patients and 370 controls in South London, examining genetic, environmental, and biological risk factors.
Why This Research Matters
This is one of the most influential studies linking high-potency cannabis to psychosis risk. The finding that 25% of cases are attributable to high-potency cannabis has directly influenced public health messaging and policy debates.
The Bigger Picture
The GAP study demonstrates that psychosis arises from the interaction of multiple genetic and environmental factors, with high-potency cannabis being a particularly important modifiable risk factor. This framing has shifted the conversation from "does cannabis cause psychosis?" to "how much psychosis does high-potency cannabis cause?"
What This Study Doesn't Tell Us
South London population may not generalize globally; cannabis potency was estimated, not measured; observational design cannot prove causation; population-attributable fraction depends on prevalence assumptions.
Questions This Raises
- ?Would reducing average cannabis potency through regulation decrease psychosis incidence?
- ?Can polygenic risk scores identify individuals for whom cannabis poses the greatest psychosis risk?
Trust & Context
- Key Stat:
- 25% of new psychosis cases attributable to high-potency cannabis use
- Evidence Grade:
- Strong: large prospective case-control study with comprehensive genetic and environmental assessment from a leading research group.
- Study Age:
- Published 2020.
- Original Title:
- The influence of risk factors on the onset and outcome of psychosis: What we learned from the GAP study.
- Published In:
- Schizophrenia research, 225, 63-68 (2020)
- Authors:
- Murray, R M(4), Mondelli, V, Stilo, S A(2), Trotta, A, Sideli, L, Ajnakina, O, Ferraro, L, Vassos, E, Iyegbe, C, Schoeler, T, Bhattacharyya, S, Marques, T R, Dazzan, P, Lopez-Morinigo, J, Colizzi, M, O'Connor, J, Falcone, M A, Quattrone, D, Rodriguez, V, Tripoli, G, La Barbera, D, La Cascia, C, Alameda, L, Trotta, G, Morgan, C, Gaughran, F, David, A, Di Forti, M
- Database ID:
- RTHC-02738
Evidence Hierarchy
Enrolls participants and follows them forward in time.
What do these levels mean? →Frequently Asked Questions
How much psychosis does cannabis cause?
The GAP study estimated that about 25% of new psychosis cases in South London were attributable to high-potency cannabis use, making it one of the most important modifiable risk factors.
Does cannabis potency matter for psychosis risk?
Yes. The study specifically linked high-potency cannabis (like skunk) to psychosis risk. Lower-potency cannabis was not associated with the same level of risk.
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Cite This Study
https://rethinkthc.com/research/RTHC-02738APA
Murray, R M; Mondelli, V; Stilo, S A; Trotta, A; Sideli, L; Ajnakina, O; Ferraro, L; Vassos, E; Iyegbe, C; Schoeler, T; Bhattacharyya, S; Marques, T R; Dazzan, P; Lopez-Morinigo, J; Colizzi, M; O'Connor, J; Falcone, M A; Quattrone, D; Rodriguez, V; Tripoli, G; La Barbera, D; La Cascia, C; Alameda, L; Trotta, G; Morgan, C; Gaughran, F; David, A; Di Forti, M. (2020). The influence of risk factors on the onset and outcome of psychosis: What we learned from the GAP study.. Schizophrenia research, 225, 63-68. https://doi.org/10.1016/j.schres.2020.01.011
MLA
Murray, R M, et al. "The influence of risk factors on the onset and outcome of psychosis: What we learned from the GAP study.." Schizophrenia research, 2020. https://doi.org/10.1016/j.schres.2020.01.011
RethinkTHC
RethinkTHC Research Database. "The influence of risk factors on the onset and outcome of ps..." RTHC-02738. Retrieved from https://rethinkthc.com/research/murray-2020-the-influence-of-risk
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.