High-potency cannabis, or frequent use, raises the risk of psychosis

ROYAL GORILLA, Girl Scout Cookies and Fat Banana are just a few of the improbably named strains of high-potency cannabis out there. In the former, levels of tetrahydrocannabinol (THC), the chemical behind the psychological effects of cannabis, are above 25%. Reviewers describe it as “overwhelming” for novices and a “beautiful euphoric couch-locking experience” for others.

However enticing that may sound, regular use of cannabis with a potency greater than 10% increases the risk of developing psychosis five-fold, according to a study published this week by the Lancet. It also found that using less potent strains daily increased the risk three-fold. Marta Di Forti, a clinical scientist at Kings College in London, and her colleagues looked at cannabis use among 901 European patients newly diagnosed with psychosis. A non-smoking control group was recruited from the general population.

The study adds substantial weight to the evidence linking cannabis to the onset of psychosis. It also suggests that differences between varieties and how often they are used could help explain why rates of psychosis among cannabis users vary across Europe.

Other factors, including genetic susceptibility, stress and injury, are also thought to be at work. Nonetheless, a growing body of evidence makes it likely that cannabis use is triggering mental-health problems in Europe. This may be particularly true in London and Amsterdam where high-THC weed is prevalent. In London, 30% of new cases of psychosis in the study were estimated to be tied to strong cannabis—or an additional 13.8 cases per 100,000 people every year.

With the relaxation of cannabis laws in the United States and Canada, many will wonder what this means for countries still developing their own policies. Given the impracticality of removing high-potency strains from the illegal market, the finds may support calls for legal, regulated sales of less harmful strains. It is difficult to know how this would affect public health. However, as one scientist remarked recently, while laboratory animals are an expensive way of understanding the risks of cannabis use, “North Americans come free.”