Do cannabis and urbanicity co-participate in causing psychosis? Evidence from a 10-year follow-up cohort study

Background. Cannabis use is considered a component cause of psychotic illness, interacting with genetic and other environmental risk factors. Little is known, however, about these putative interactions. The present study investigated whether an urban environment plays a role in moderating the effects of adolescent cannabis use on psychosis risk. Method. Prospective data (n = 1923, aged 14-24 years at baseline) from the longitudinal population-based German Early Developmental Stages of Psychopathology cohort study were analysed. Urbanicity was assessed at baseline and defined as living in the city of Munich (1562 persons per km(2); 4061 individuals per square mile) or in the rural surroundings (213 persons per km(2); 553 individuals per square mile). Cannabis use and psychotic symptoms were assessed three times over a 10-year follow-up period using the Munich version of the Composite International Diagnostic Interview. Results. Analyses revealed a significant interaction between cannabis and urbanicity [10.9% adjusted difference in risk, 95% confidence interval (CI) 3.2-18.6, p = 0.005]. The effect of cannabis use on follow-up incident psychotic symptoms was much stronger in individuals who grew up in an urban environment (adjusted risk difference 6.8%, 95% CI 1.0-12.5, p = 0.021) compared with individuals from rural surroundings (adjusted risk difference -4.1%, 95% CI -9.8 to 1.6, p = 0.159). The statistical interaction was compatible with substantial underlying biological synergism. Conclusions. Exposure to environmental influences associated with urban upbringing may increase vulnerability to the psychotomimetic effects of cannabis use later in life.