A study by Robin Murray and Wayne Hall, researchers in the UK and Australia, published last April 2020, posed the question: Will Legalization and Commercialization of Cannabis Use Increase the Incidence and Prevalence of Psychosis? This is what they found:

“Other countries have decriminalized the drug. In Holland, cannabis can be bought in designated cafes, and in Portugal, the police refer those who regularly use cannabis for counseling. Psychiatrists have played a prominent role in the debate over the health consequences of legalization in many countries, especially in the UK, but the public debate in the US has been notable for the absence of input from psychiatrists”.2

“Growing evidence suggests that this is also the case for cannabis and psychosis. Boydell et al8 showed that the incidence of schizophrenia doubled in London, England, between 1965 and 1999 and attributed much of this to the increased use of cannabis. Hjorthøj et al9 demonstrated that the incidence of cannabis-induced psychosis more than doubled in Denmark between 2006 and 2016. Gonçalves-Pinhoetal10 reported that rates of hospitalization for psychotic disorders in Portugal increased 29-fold in the 15 years after decriminalization; the percentage of patients with a psychotic disorder and documented cannabis use rose from 0.87% to 10.60%”.

“Governments that decide to legalize cannabis should use some of the tax revenue to monitor cannabis price, consumption, and potency levels and to carefully evaluate the long-term repercussions for mental health in different US states and Canadian provinces. “.

“However, the population attributable fraction was greatest in the 2 cities with the highest-potency cannabis, London and Amsterdam, where it was estimated that 30% and 50% of new cases of psychosis, respectively, would be prevented if no one smoked high-potency cannabis”.

Editor’s Note: high-potency cannabis is defined at THC >10% potency

Read the full science research study in JAMA Psychiatry.