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The strength of marijuana could be an important consideration when evaluating the health risks of using cannabis, according to a study of more than 2,000 patients and controls published yesterday (March 19) in The Lancet Psychiatry.

At 10 locations in Europe and another in Brazil, researchers found that variation in the incidence of psychotic disorder correlated with differences in the frequency of cannabis use and, in particular, the use of high-potency cannabis, which can carry levels of psychoactive tetrahydrocannabinol (THC) greater than 10 percent.

“Daily use of high-potency cannabis and how this varies across Europe explains some of the striking variations we have measured in the incidence of psychotic disorder,” coauthor Marta Di Forti of King’s College London tells The Guardian.

After controlling for education, drinking habits, and other factors, the researchers found that there was a 40 percent greater chance of developing a psychotic disorder among people who had used the drug more than once a week, as compared with those who had rarely or never tried cannabis. The risk was three times greater among daily users, as compared with rare or nonusers, and higher still among daily users of high-potency cannabis.

The link between daily use of cannabis and psychosis was strongest in Amsterdam, where cannabis is sold in coffee shops and is almost always quite potent, with varieties containing up to 67 percent THC. The researchers estimated that if high-potency versions of the drug were not available, half of first-time cases of psychotic disorders in the city could be avoided.

“If you are going to legalise cannabis, unless you want to pay for more a lot more psychiatric beds and a lot more psychiatrists, then you need to devise a system where you would legalise in a way that wouldn’t increase the consumption and increase the potency,” coauthor Robin Murray, from King’s College London, tells The Guardian.