Alcohol confounds relationship between cannabis misuse and psychosis conversion in a high-risk sample

ObjectiveCannabis use has been examined as a predictor of psychosis in clinical high-risk (CHR) samples, but little is known about the impact of other substances on this relationship. MethodSubstance use was assessed in a large sample of CHR participants (N=370, mean age=18.3) enrolled in the multisite North American Prodrome Longitudinal Study Phase 1 project. Three hundred and forty-one participants with cannabis use data were divided into groups: No Use (NU, N=211); Cannabis Use without impairment (CU, N=63); Cannabis Abuse/Dependence (CA/CD, N=67). Participants (N=283) were followed for 2years to determine psychosis conversion. ResultsAlcohol (45.3%) and cannabis (38.1%) were the most common substances. Cannabis use groups did not differ on baseline attenuated positive symptoms. Seventy-nine of 283 participants with cannabis and follow-up data converted to psychosis. Survival analysis revealed significant differences between conversion rates in the CA/CD group compared with the No Use (P=0.031) and CU group (P=0.027). CA/CD also significantly predicted psychosis in a regression analysis, but adjusting for alcohol use weakened this relationship. ConclusionThe cannabis misuse and psychosis association was confounded by alcohol use. Non-impairing cannabis use was not related to psychosis. Results highlight the need to control for other substance use, so as to not overstate the cannabis/psychosis connection.