The Influence of Cannabis Use on the Incidence of Psychotic Disorders
An important study on the influence of cannabis use on the incidence of psychotic disorders was just published in the May issue of the Lancet Psychiatry (Di Forti et al., Lancet Psychiatry 2019; 6: 427-436). The study was part of the large transnational case-control study (EU-GEI) that has reported last year a striking eight-times difference in the incidence of psychotic disorder across 16 European sites (from England, France, Italy, The Netherlands, Spain) plus one in Brazil (Jongsma et al., JAMA Psychiatry 2018; 75: 36-46).
901 subjects with their first episode of psychosis were now recruited from 11 of those 17 sites. They were matched with 1237 controls from the general population representative of the respective area. It should be emphasized that the authors used a broad definition of psychosis that included major depression (ICD-10 diagnoses F20 – F33). These cases and controls were characterized with regard to their cannabis consumption. They were classified according to the frequency of their cannabis use (never, rare, more than once a week, daily) and the content of tetrahydrocannabinol (THC) in the cannabis used (low potency: < 10% THC vs. high potency cannabis: ≥10% THC). The paper can be openly accessed here.
Both frequency and THC content had a profound influence on the incidence (= probability of occurrence of an event in a population within a specified period of time) of psychosis. Daily consumption of high potency cannabis increased the risk of psychosis almost five-fold compared to never-use (which was used as the reference = 1). Daily use of low-potency cannabis increased the risk more than twofold. These risk ratios did not significantly differ between sites. However, there were large differences in daily cannabis use and consumption of high-potency cannabis, both being highest in Amsterdam.
When the authors of this study calculated the influence of type and frequency of cannabis use relative to usage patterns in different European areas (with a very high frequency of daily use of high-potency cannabis in Paris, London and especially Amsterdam), they arrive at striking implications: 20% of new cases of psychotic disorder across all sites could have been prevented if daily use of cannabis had been abolished. If high-potency cannabis were no longer accessible, the adjusted incidence rates for all psychotic disorder in Amsterdam would drop by approximately 50% (!) and in London by one third.
The authors come to the following important conclusion: „Our findings confirm previous evidence of the harmful effect on mental health of daily use of cannabis, especially of highpotency types. Importantly, they indicate for the first time how cannabis use affects the incidence of psychotic disorder. Therefore, it is of public health importance to acknowledge alongside the potential medicinal properties of some cannabis constituents the potential adverse effects that are associated with daily cannabis use, especially of high potency varieties.“
In the debate about the legalization of cannabis we should be aware of its risks, and we should use it responsibly.