A new Irish study shows that young people who use multiple substances are significantly more likely to experience mental health difficulties than their peers who do not use drugs, alcohol, or nicotine. In this study, which has been published in the Irish Journal of Psychological Medicine,1 researchers from the Royal College of Surgeons in Ireland (RCSI) University of Medicine and Health Sciences and collaborating institutions examined data from 4,404 students aged 15–16 years participating in the programme Planet Youth Ireland. The research investigated how the use of cannabis, alcohol, and nicotine, either individually or in combination, was associated with a range of negative mental health outcomes, including anxiety, depression, behavioural difficulties, and psychotic experiences.

The findings reveal a clear dose–response relationship: as the number of substances used increased, so did the likelihood of the user experiencing poor mental health. Adolescents who had used all three substances in the month prior to the study had more than five times greater odds of having general psychopathology and almost six times greater odds of having externalising behavioural problems, such as conduct difficulties and hyperactivity, compared with young people who had not used any substances during the previous month. They were also more than twice as likely to experience internalising problems, including symptoms of anxiety and depression, and more than three times as likely to report psychotic experiences.

The study found that polysubstance use was relatively common among adolescents. While approximately 60% of participants reported no recent substance use, 22% had used one substance during the previous month, 12% had used two substances, and nearly 5% had used cannabis, alcohol, and nicotine together. More than one-half of adolescents who reported using all three substances met the criteria for at least one clinically significant measure of psychopathology. The study also noted differences in outcomes by sex; although increased mental health risks were observed across all groups, female adolescents who engaged in polysubstance use consistently experienced the poorest mental health outcomes. Girls using all three substances demonstrated particularly strong associations with general psychopathology, psychotic experiences, and externalising behavioural problems, suggesting that young females who engage in polysubstance use may represent a particularly vulnerable population requiring targeted support.

The researchers emphasise that the study cannot determine whether substance use causes mental health problems or whether existing psychological difficulties increase the likelihood of substance use. Instead, the findings suggest a complex, potentially bidirectional relationship in which substance use and mental health difficulties may reinforce one another. Nevertheless, the authors suggest that the results of the study may have important implications for schools, healthcare providers, and policy-makers. Notably, rather than focusing on single substances in isolation, prevention programmes and clinical services should routinely assess polysubstance use when supporting young people. Likewise, mental health services should recognise multiple substance use as a potential marker of underlying psychological distress.

1         Fleury R, Dooley N, Staines L, et al. (2026) Adolescent polysubstance use and psychopathology: a population-based survey in schools. Ir J Psychol Med [Early online]: 1–10. Available from: https://www.drugsandalcohol.ie/45705/