Aims To examine the longitudinal associations in both directions between mental health and substance use in adolescence.
Design Three-year longitudinal cohort.
Setting Britain (nationally representative sample).
Participants 3607 youths aged 11–16 years at baseline.
Measurements Externalizing and internalizing mental health problems were measured using brief questionnaires (parent-reported Strengths and Difficulties Questionnaire) and diagnostic interviews, including clinician-rated diagnoses of mental disorder. Substance use was measured by youth self-report, and included regular smoking, frequent alcohol consumption, regular cannabis use and ever taking other illicit drugs.
Findings Externalizing (specifically behavioural) problems at baseline independently predicted all forms of substance use, with a particularly strong effect on smoking. In all cases this association showed a dose–response relationship. In contrast, although internalizing problems had a strong univariable association with smoking, this disappeared after adjusting for comorbid externalizing problems. There was little or no evidence that baseline substance use predicted mental health at follow-up.
Conclusions Externalizing problems predict adolescent substance use, and adjusting for comorbid externalizing problems is vital when investigating the effects of internalizing problems. A dose–response effect of externalizing problems is seen across the full range. Programmes seeking to prevent adolescent substance use by reducing externalizing problems may therefore wish to consider population-wide interventions rather than targeting individuals only at the negative extreme.