BACKGROUND AND AIMS Child maltreatment (CM) is associated with several negative mental health outcomes in later life, including alcohol and substance use (AU and SU). Internalising (e.g. anxiety and depression) and externalising (e.g. delinquency and anti-social behaviour) behaviours have also been associated with CM, and with AU and SU. This study measured whether externalising or internalising behaviours in adolescence mediate the relationship between agency-reported CM and hospital admissions for AU or SU.

DESIGN, SETTING AND PARTICIPANTS Observational study linking administrative health data from Queensland, Australia, to prospective birth cohort data comprising agency-reported CM up to 14 years (exposure).

MEASUREMENTS Externalising and internalising behaviours at 14 years measured using the Youth Self-Report (mediator) and administratively linked inpatient hospital admissions for AU and SU from ages 25-39 (outcome).

FINDINGS Adjusted causal mediation analyses (n = 5092) found that externalising behaviours statistically significantly mediated 31% (P = 0.007) of the CM AU relationship, and 22% of the CM SU relationship (P = 0.016). Internalising behaviours did not statistically significantly mediate between CM and either AU or SU.

CONCLUSIONS In Queensland, Australia, externalising behaviours appear to partially mediate the relationship between agency-reported child maltreatment and hospital admissions for alcohol and substance use, while internalising behaviours do not.