Twin Study: Adolescent Cannabis Exposure Not an Independent Cause of Psychosis in Adulthood

Minneapolis, MN: Cannabis exposure during adolescence is not independently associated with either adult-onset psychosis or signs of schizophrenia, according to longitudinal data published in the Journal of Abnormal Psychology

Researchers affiliated with the University of Minnesota Institute of Child Development assessed the relationship between adolescent cannabis use and adult-onset psychosis in a longitudinal co-twin control analysis. Scientists identified no dose-response relationship in models that compared the greater cannabis using twin to the lesser using co-twin with respect to psychosis-proneness in adulthood. They also reported no differing effects on subjects’ levels of cannabis exposure and their later risk of schizophrenia.

Researchers reported: “Epidemiological studies have repeatedly shown that individuals who use cannabis are more likely to develop psychotic disorders than individuals who do not. It has been suggested that these associations represent a causal effect of cannabis use on psychosis, and that psychosis risk may be particularly elevated when use occurs in adolescence. … This study, however, does not support these hypotheses, suggesting instead that observed associations are more likely due to confounding by common vulnerability factors.”

They concluded, “[T]he results suggest this association is likely attributable to familial confounds rather than a causal effect of cannabis exposure. … Our results suggest that the threat of potential harm to adolescents via meaningful increases in risk of long-term psychotic illness may be overstated. … Thus, clinical and public health interventions aimed at decreasing the prevalence and burden of psychotic illnesses may benefit from focusing their attention elsewhere.”

Full text of the study, “Adolescent cannabis use and adult psychoticism: A longitudinal co-twin control analysis using data from two cohorts,” appears in the Journal of Abnormal Psychology. Additional information on cannabis use and mental health outcomes is available from NORML.