Association of Combined Patterns of Tobacco and Cannabis Use in Adolescence With Psychotic Experiences

ImportanceThere has been increasing concern about potentially causal effects of tobacco use on psychosis, but epidemiological studies have been less robust in attempts to minimise effects of confounding than studies of cannabis use have been.ObjectiveTo examine the association of patterns of cigarette and cannabis use with preceding and subsequent psychotic experiences, and compare patterns of confounding across these patterns.Design, Setting, and ParticipantsA cohort study of adolescents from the Avon Longitudinal Study of Parents and Children birth cohort initially consisting of 14,062 children. Data were collected periodically from September 6, 1990, with collection ongoing, and analyzed from August 8, 2016 to June 14, 2017. Cigarette and cannabis use data were summarised using longitudinal latent class analysis to identify longitudinal classes of substance use, and associations between classes and psychotic experiences at 18 years were assessed.ExposuresDepending on the analysis model, exposures were longitudinal classes of substance use or psychotic experiences at age 12 years.Main Outcomes and MeasuresLogistic regression was used to examine the relationships between substance use longitudinalclasses and subsequent onset of psychotic experiences.ResultsLongitudinal classes were derived using 5,300 (56.1% female) individuals who had at least 3measures of cigarette and cannabis use between ages 14-19 years. Prior to adjusting for a range of potential confounders, there was strong evidence that early-onset cigarette-only use (4.3%), earlyonset cannabis use (3.2%), and late-onset cannabis use (11.9%), but not later-onset cigarette-only use (14.8%) latent classes were associated with increased psychotic experiences compared to nonusers(65.9%) (omnibus P<0.001). After adjusting for confounders, the association for early-onset cigarette-only use attenuated substantially (unadjusted odds ratio (OR) = 3.03, 95%CI 1.13, 8.14; adjusted OR = 1.78, 95%CI 0.54, 5.88), whereas those for early-onset (adjusted OR = 3.70, 95%CI 1.66, 8.25) and late-onset (adjusted OR = 2.97, 95%CI 1.63, 5.40) cannabis use were unchanged.Conclusions and relevanceOur findings indicate that whilst individuals who use either cannabis or cigarettes duringadolescence have an increased risk of developing subsequent psychotic experiences, theepidemiological evidence for this being causal is substantively more robust for cannabis than it is for tobacco.