LONDON — A global review published in Lancet Psychiatry analyzed cannabis policy changes, user numbers, drug potency, and psychosis rates between 2000 and 2025. The review indicated that in the U.S. and Canada, the establishment of commercial cannabis markets correlated with increased user numbers and higher drug potency. Hospital visits for psychosis and other mental health issues linked to cannabis increased in regions with commercial cannabis markets.

Tom Freeman, a professor of psychology at the University of Bath and the first author on the review, said: "When we look globally, there is a shift towards more liberal cannabis policies. This provides an opportunity to evaluate outcomes before and after policy implementation regarding cannabis use, addiction, and related psychiatric disorders."

Decriminalization of cannabis in Europe, Africa, Oceania, and Asia showed little evidence of changes in usage or psychiatric illness rates. Freeman said: "It is still early to assess these policy changes when considering the long history of alcohol and tobacco use, but current data indicates this direction of development." He added: "When there is a for-profit industry, particularly for an addictive product, there is an incentive to sell cheaper and high potency products because increased sales generate higher revenue, which can lead to increased use."

Sir Robin Murray, a professor of psychiatric research at King’s College London, said: "Unfortunately, in North America, legalization has been followed by commercial expansion facilitated by advertising from companies seeking to sell cannabis at levels comparable to tobacco at its peak. It remains unclear whether legalization can occur without commercialization in western capitalist economies, as the expanding cannabis industry has so far prevented this outcome."

Alex Stevens, a professor of criminology at the University of Sheffield, said: "Research consistently indicates that restricted regulatory models and decriminalization for possession do not automatically increase usage. Reform opponents claim liberalization would cause usage to rise significantly, but available country data does not support this assertion." In Uruguay, cannabis was legalized in 2013 under a strictly state-controlled market. Registered adults can purchase cannabis from pharmacies, participate in cannabis social clubs, or cultivate the plant at home.

In the U.K., cannabis is classified as a class B drug, and possession carries a maximum penalty of up to five years in prison, an unlimited fine, or both. The London Drugs Commission, which was commissioned by London mayor Sadiq Khan, published a report proposing the decriminalization of cannabis possession for recreational use. The report found that Black people were five times more likely to be stopped and searched on suspicion of drug-related offenses than white people.