Alcohol Confounds Relationship Between Cannabis Misuse and Psychosis Conversion in a High-Risk Sample

Acta Psychiatr Scand. 2015 Jul;132(1):60-8. doi: 10.1111/acps.12382. Epub 2015 Jan 9.

Abstract

Objective: Cannabis 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.

Method: Substance 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 ≥2 years to determine psychosis conversion.

Results: Alcohol (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.

Conclusion: The 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.

Keywords: alcohol use; cannabis use; clinical high-risk; prodromal states; psychosis; substance use.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Adolescent
  • Alcohol-Related Disorders / epidemiology*
  • Alcohol-Related Disorders / psychology
  • Causality
  • Comorbidity
  • Disease Progression
  • Female
  • Humans
  • Male
  • Marijuana Abuse / epidemiology*
  • Marijuana Abuse / psychology
  • Psychoses, Substance-Induced / epidemiology*
  • Psychoses, Substance-Induced / psychology
  • Psychotic Disorders / epidemiology*
  • Psychotic Disorders / psychology
  • Risk Assessment
  • Risk Factors
  • Risk-Taking*
  • United States / epidemiology
  • Young Adult