Researchers reporting in the Journal of the American Medical Association (JAMA) have found that both adolescent cannabis use and cigarette use are associated with increased risk of psychosis. The association was greater for cannabis, according to their study, published online this month and available for download here.
Design, Setting and Participants: This cohort study used data from the Avon Longitudinal Study of Parents and Children, which initially consisted of 14, 062 children. Data were collected periodically from September 6, 1990, with collection ongoing, and analyzed from August 8, 2016, through June 14, 2017. Cigarette and cannabis use data were summarized using longitudinal latent class analysis to identify longitudinal classes of substance use. Associations between classes and psychotic experiences at age 18 years were assessed.
Discussion: Associations between genetic risk for psychosis and both cannabis use and heaviness of cigarette use are also consistent with causal effects, reverse-causal effects, and pleiotropy explanations.
As most people who use cannabis also smoke cigarettes, teasing out potentially causal effects of cannabis from those of tobacco is difficult, particularly as individuals usually mix their cannabis with tobacco, even when classing themselves as nonsmokers.