Major London study: one-quarter of new psychosis cases attributed to high-potency cannabis | Research

The takeaway

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.

Psychiatrists, public health officials, cannabis policy makers, and geneticists studying psychosis risk.

25% of new psychosis cases attributable to high-potency cannabis use

What the researchers 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.

Why it 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 numbers in context

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 the study worked

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.

What this study cannot 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.

How to read the evidence

Strong: large prospective case-control study with comprehensive genetic and environmental assessment from a leading research group.

When this study was published

Published 2020.

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?"

Questions still open

Would reducing average cannabis potency through regulation decrease psychosis incidence? Can polygenic risk scores identify individuals for whom cannabis poses the greatest psychosis risk?

Common 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.

Read the original research

The influence of risk factors on the onset and outcome of psychosis: What we learned from the GAP study.

Schizophrenia research, 225, 63-68

View on PubMed ↗Publisher’s paper ↗

Citation

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