Studies have shown that THC can cause or worsen schizophrenia, especially in people who use it problematically. A Danish team notes an increase in the incidence of schizophrenia in this country between 2000 and 2012 and notes that the fraction of risk attributable to a cannabis use disorder has tripled or even quadrupled over the past twenty years, paralleling the concentration of THC.
To clarify the characteristics of the links between cannabis use and schizophrenia, this team analyzed data from national registries covering nearly 7 million people aged 16-49 between 1972 and 2021. In total, there were reported 45,327 cases of schizophrenia during follow-up, covering 129 million person-years.
15% of new cases of schizophrenia could have been prevented in men
The data support a stronger association between cannabis use disorder and schizophrenia in men than in women (adjusted HR 2.42; 95% confidence interval 2.33 to 2.52 for the former and 2.02; from 1.89 to 2.17 for the second). In the 16- to 20-year-old population, the risk is doubled for boys. The authors note that the share of risk attributable to cannabis in developing schizophrenia increases by an average of 4.8% per year for men and 3.2% for women.
Thus, in 2021, 15% of new cases of schizophrenia in men could have been prevented in the absence of problem cannabis use, and 4% in women. This proportion attributable to a cannabis use disorder varies according to age, and would reach 25 to 30% among men aged 21 to 30, perhaps even as late as their 40s.
Of course, problem cannabis use is not responsible for most cases of schizophrenia. However, it contributes a non-negligible share of its incidence, a share that has been steadily increasing for several decades.
The increasing number of countries that have legalized the non-medical use of cannabis, combined with the increase in THC concentration, increases the risk of use disorder by reducing public perception of the dangers involved. For the authors, this study adds to the growing evidence suggesting that cannabis use is a modifiable risk factor for schizophrenia, thus underscoring the need for evidence-based strategies to regulate use and effectively prevent both the disorders from use than schizophrenia.
(Roseline Peluchon su Jim – International Journal of Medicine of 05/19/2023)
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