Evidence suggests a causal link between cannabis use and psychotic disorder. It's unclear, however, whether patterns of cannabis use affect rates of psychotic disorder at a population level. Using data from a case-control study of first-episode psychosis at 16 sites in Europe and one in Brazil, researchers aimed to identify the patterns of cannabis use associated with the strongest odds of psychosis. They also sought to determine whether differences in those patterns contributed to variations in the incidence of psychotic disorders.

The researchers identified all patients 18 to 64 years old who presented with a first episode of psychosis to mental health services at the 17 sites during a five-year period. They recruited controls representative of the local population, excluding those with a history of psychotic disorder.

Participants completed a questionnaire about their use of cannabis and other recreational drugs. Cannabis potency was classified as high or low, depending on the concentration of delta-9-tetrahydrocannabinol (THC). The researchers analyzed 901 cases of first-episode psychosis and 1,237 controls. They found that daily use of cannabis increased a person's odds of developing a psychotic disorder compared with never having used it. Daily use of high-potency cannabis had more than a four times higher risk of psychotic disorder compared with never having used cannabis. In areas in which daily use of cannabis and use of high-potency cannabis were more prevalent in the general population, the researchers note, there were more cases of psychotic disorder.

The study had several limitations, including that data on cannabis use were not validated by biologic measures. In addition, the researchers note that their potency measure was based only on the amount of THC and didn't include the proportion of cannabidiol (CBD). The authors advise that the potential health benefits of cannabis should be considered within the context of the potential adverse effects of daily use, particularly of high-potency cannabis.

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