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

• Published in: Psychological medicine 2011-10, Vol.41 (10), p.2121-2129
• Main Authors: Kuepper, R., van Os, J., Lieb, R., Wittchen, H.-U., Henquet, C.
• Format: Article
• Language: English
• Subjects: Adolescent; Biological and medical sciences; Cannabis; Child & adolescent psychiatry; Cohort analysis; Drug use; Environmental aspects; Female; Follow-Up Studies; Germany - epidemiology; Humans; Interview, Psychological; Logistic Models; Male; Marijuana; Marijuana Abuse - diagnosis; Marijuana Abuse - epidemiology; Marijuana Abuse - psychology; Medical sciences; Mental disorders; Psychology. Psychoanalysis. Psychiatry; Psychopathology. Psychiatry; Psychoses; Psychosis; Psychotic Disorders - diagnosis; Psychotic Disorders - epidemiology; Psychotic Disorders - psychology; Risk Factors; Rural communities; Rural Population - statistics & numerical data; Urban areas; Urban environment; Urban Population - statistics & numerical data; Young Adult
• ISSN: 0033-2917; 1469-8978
• DOI: 10.1017/S0033291711000511

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. 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 km2; 4061 individuals per square mile) or in the rural surroundings (213 persons per km2; 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. 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. Exposure to environmental influences associated with urban upbringing may increase vulnerability to the psychotomimetic effects of cannabis use later in life.