Decriminalising cannabis possession or tightly regulating access does not appear to increase how many people use the drug, according to a major international review — but opening commercial markets does, alongside rises in potency and cannabis-related mental health presentations.
Published in Lancet Psychiatry, the study analysed how cannabis policy shifted globally between 2000 and 2025, tracking changes in use, product strength and rates of psychosis before and after reforms took effect.
Commercial sales tell a different story
In countries that developed for-profit cannabis markets, including parts of the United States and Canada, researchers recorded more users, stronger products and increased hospital visits linked to psychosis and other cannabis-related mental health conditions.
By contrast, jurisdictions that decriminalised possession across Europe, Africa, Oceania and Asia showed little evidence of rising use or psychiatric illness. A similar pattern was observed in Uruguay, where cannabis is legal but supply is tightly controlled by the state rather than driven by commercial retail.
"When we look globally, there's been a rapid shift towards more liberal cannabis policies," said Tom Freeman, professor of psychology at the University of Bath and the review's lead author. "That gives us a chance to evaluate what happened from before to after, in terms of cannabis use, cannabis addiction and psychiatric disorders related to cannabis."
The profit incentive
Freeman said the findings were encouraging for policymakers weighing decriminalisation or strictly regulated access, but warned that open commercial markets behaved very differently.
"When there's a for-profit industry, particularly for an addictive product, there's an incentive to sell cheaper products and high-potency products, because the more they sell, the more money they'll make — and that can increase use," he said, drawing parallels with long-standing challenges regulating tobacco and alcohol.
Sir Robin Murray, professor of psychiatric research at King's College London, said commercialisation of supply rather than legalisation itself appeared to be the critical factor.
"Unfortunately, in North America, legalisation has been followed by rampant commercialisation facilitated by advertising from companies which would like to sell as much cannabis as tobacco at its peak," Murray said. "Is it possible to legalise without commercialising in western capitalist countries? It should be possible, but so far no country has managed this."
What it means for UK debate
The review lands amid renewed discussion of cannabis policy in Britain. Cannabis remains a Class B drug, with possession without a prescription carrying penalties of up to five years in prison, an unlimited fine, or both.
Last year, the London Drugs Commission — commissioned by Mayor Sadiq Khan — argued that existing sanctions for personal possession were unjustified and recommended decriminalisation for recreational use. The report also highlighted that Black people were five times more likely to be stopped and searched for suspected drug offences than white people.
Alex Stevens, professor of criminology at the University of Sheffield, described the review as essential reading for policymakers. "We're getting a fairly consistent message from the research that restricted models of regulation and decriminalisation for possession don't necessarily lead to increases in use," he said. "That is what opponents of these reforms always say — but that's not what we see in the countries that have done it."
Reporting based on findings published in Lancet Psychiatry, June 2026.
