International scientists have analyzed major shifts in cannabis policy from 2000 to 2025, including rates of use, cannabis potency, and the incidence of psychotic disorders after new regulations took effect. The findings were published in the journal Lancet Psychiatry.
The study found that in places that opened commercial markets for cannabis, such as the United States and Canada, usage increased along with the potency of the substance. The number of hospital admissions for cannabis-related psychosis and other mental health problems also rose.
By contrast, in regions of Europe, Africa, Oceania and Asia that only decriminalized possession of cannabis, there was no clear evidence of a change in usage levels or mental illness. Similar patterns were seen in Uruguay, where cannabis is legal but tightly state-controlled.
Tom Freeman, professor of psychology at the University of Bath and the study's lead author, said: “Globally, we are seeing a rapid shift toward more liberal policies. This research offers an encouraging message for policy-makers considering decriminalization or tight regulation of cannabis.”
Freeman stressed the difference made by an open market where cannabis is sold much like alcohol and tobacco: “When there is a for-profit industry, especially with addictive products, there is always an incentive to sell cheaper, higher-potency products. This boosts revenue but also causes more harm.”
In the UK, cannabis is a class B drug and possession can lead to up to five years in prison. However, a 2025 report by the London Drugs Commission (under mayor Sadiq Khan) argued that these measures are disproportionate and proposed decriminalizing possession of cannabis for recreational use, because Black people are five times more likely than white people to be searched on suspicion of drug offenses.
Uruguay became the first country to fully legalize cannabis in 2013, but the market is tightly state-controlled. Registered adults can buy cannabis from pharmacies, through social clubs, or grow it at home.
Professor Robin Murray of King’s College London commented: “In North America, legalization brought rampant commercialization with advertising from companies eager to sell as much as possible. Is it possible to legalize without commercialization in Western capitalist countries? In theory yes, but so far no country has managed it because of the power of the billion-dollar cannabis industry.”
Alex Stevens, professor of criminology at the University of Sheffield, called the research “very needed” for policy-makers. “Critics always argued that if you allow use, the rates will skyrocket, but in reality in all the countries that have done it, we don’t see that happening.”
