Daily High-Potency Cannabis Use and Psychosis Risk: The Largest European Study Drew a Direct Line
The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI): a multicentre case-control study
Daily use of high-potency cannabis (10%+ THC) was linked to nearly 5 times higher odds of first-episode psychosis, and the effect varied dramatically by city.
In London, you can buy cannabis that regularly exceeds 20% THC from a dealer down the street. In Bologna, the hash available is closer to 5% THC. Both cities have cannabis users. But London has substantially more people developing psychosis for the first time.
Marta Di Forti wanted to know if that difference was a coincidence.
Her team recruited patients experiencing first-episode psychosis across eleven cities in six European countries and Brazil, alongside population controls from the same cities. What emerged was the largest study ever to connect cannabis potency at the market level with psychosis incidence at the population level. The numbers were stark enough to shift policy conversations across Europe.
The EU-GEI Study
The European Network of National Schizophrenia Networks Studying Gene-Environment Interactions (EU-GEI) was designed to answer a question that single-city studies couldn't: does the type of cannabis available in a city affect how many people in that city develop psychosis?
The Numbers
Daily use of high-potency cannabis (10%+ THC) was associated with nearly five times the odds of experiencing a first psychotic episode compared to people who had never used cannabis. Daily use of any potency carried about three times the odds. Less-than-weekly use carried no significantly increased risk.
But the finding that distinguished this study from everything before it was the city-level analysis.
The Geographic Pattern
Amsterdam — where high-potency cannabis has dominated the coffeeshop market for decades — had the highest estimated attributable fraction: 50.3%. If high-potency cannabis were no longer available, the researchers estimated that half of new psychosis cases in the city could be prevented. London followed at 30.3%. Cities with lower-potency markets had lower attributable fractions.
The correlation between prevalence of high-potency cannabis use among controls and the incidence rate of psychosis across the eleven sites was striking (r = 0.7, p = 0.03). Cities where more people used potent cannabis had more psychosis.
r = 0.7
correlation between prevalence of daily high-potency cannabis use in a city's population and that city's incidence rate of psychotic disorder — across eleven sites in Europe and Brazil.
This ecological correlation doesn't prove causation, but it shows something individual-level studies cannot: the cannabis market itself — specifically what potency is available — appears to shape population-level psychosis rates. This is a public health finding, not just a clinical one.
Di Forti et al. (2019), Lancet Psychiatry
The Causal Debate
The study calculated population attributable fractions (PAFs) — a measure that assumes a causal relationship between exposure and outcome. This assumption drew immediate criticism.
Suzi Gage and Matthew Hickman published a letter in The Lancet Psychiatry titled "High-potency cannabis and incident psychosis: correcting the causal assumption." They argued that calculating how many psychosis cases could be "prevented" by removing high-potency cannabis presupposes the very causal link the study was trying to establish.
“The authors present population attributable fractions, but these assume a causal effect that has not been established. The fundamental question — whether cannabis causes psychosis or shared genetic and environmental factors explain the association — remains open.”
— Suzi Gage & Matthew Hickman
University of Liverpool / University of Bristol
Letter to The Lancet Psychiatry criticizing the causal framing of the Di Forti study
Di Forti's team replied thoughtfully: they acknowledged that psychosis has complex, multifactorial origins and that they do not believe cannabis is the sole cause. But they argued that PAFs are standard epidemiological tools used for other risk factors (smoking and lung cancer, for example), and that waiting for perfect causal proof before acting on a clear dose-response signal would be irresponsible public health.
The Genetic Vulnerability Layer
This study's findings make more sense when combined with Di Forti's earlier genetic work. In 2012, her team showed that individuals carrying specific variants of the AKT1 gene — involved in dopamine signaling in the striatum — had up to 7-fold increased risk of psychosis when using cannabis daily.
This gene-environment interaction reframes the question from "does cannabis cause psychosis?" to "does cannabis cause psychosis in genetically susceptible people who use high-potency products daily?" The answer to that second, more precise question is almost certainly yes — though proving it definitively remains difficult.
What This Means for Policy
The study landed in the middle of European cannabis legalization debates and provided ammunition for a specific policy approach: potency regulation.
Myth vs. Reality
Cannabis causes schizophrenia.
Cannabis — specifically daily use of high-potency products — is a risk factor for psychosis in vulnerable individuals, not a sufficient cause of schizophrenia. Most people who use cannabis daily, even high-potency products, never develop psychosis. But for the subset with genetic vulnerability (including AKT1 risk variants and family history), the risk multiplication is real and clinically significant. The molecule matters: THC increases psychosis risk while CBD may be protective.
The Evidence
Di Forti EU-GEI study: 5x odds with daily high-potency use. But absolute risk remains low — the vast majority of daily users never develop psychosis. AKT1 gene-environment interaction shows up to 7x risk in carriers. CBD has shown antipsychotic properties in clinical trials (Leweke 2012, McGuire 2018).
Di Forti et al. (2019); Di Forti et al. (2012); Leweke et al. (2012); McGuire et al. (2018)
If the relationship is causal — and the weight of evidence increasingly supports at least a partial causal role — then THC potency caps in legal markets could meaningfully reduce psychosis incidence. Some European countries have implemented or discussed exactly this. The study also strengthens the argument for requiring CBD content alongside THC in commercial products, since CBD appears to have antipsychotic properties that may counterbalance THC's psychotogenic effects.
For individuals, the practical implications are clear: daily use of high-potency cannabis carries a meaningful risk of psychosis, especially for those with family history of psychotic disorders. Less-than-weekly use carries no significantly increased risk. Potency and frequency are the variables that matter most.
Frequently Asked Questions
Cite this study
Di Forti, Marta; Quattrone, Diego; Freeman, Tom P.; Tripoli, Giada; Gayer-Anderson, Charlotte; Quigley, Harriet; et al.. (2019). The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI): a multicentre case-control study. The Lancet Psychiatry, 6(5), 427-436.