BACKGROUND: Substances, such as cannabis, alcohol, and nicotine, have each been associated with an increased risk of mental health concerns in adolescence. These substances are often taken concurrently and less is known about the effects of polysubstance use. We investigated the relationship between polysubstance use and psychopathology in adolescence including internalising, externalising behaviours, and psychotic experiences (PE). As a secondary analysis, we examined gender differences.

METHODS: A sample of 4,404 15-16-year-old adolescents in 40 schools in Ireland was examined. Our exposure was single substance use (cannabis, alcohol, and nicotine) and polysubstance use within the past month. The outcome of interest was psychopathology, measured using the Strengths & Difficulties Questionnaire, and the Adolescent Psychotic Symptoms Scale. Associations were explored using mixed-effects logistic models.

RESULTS: Adjusted odds ratios were calculated to examine the associations between polysubstance use and each outcome, compared to substance-naive participants. A significant dose response relationship was observed, with increasing substance use associated with higher odds of general psychopathology (aOR range: 1.65-5.09), internalising behaviours (aOR range: 1.18-2.06), externalising behaviours (aOR range: 1.68-5.79), and PE (aOR range: 1.60-3.13). Females engaging in polysubstance use exhibited the poorest mental health outcomes.

CONCLUSION: This study highlights the strong associations between adolescent polysubstance use and emerging mental health problems. The findings reinforce the need for mental health services to take polysubstance use into consideration during intervention. Tailored public health messaging is required, acknowledging polysubstance use as a marker for poor mental health among adolescents and young adults.