Associations and mediating factors between adverse childhood experiences and substance use behaviours in early adulthood: A population-based longitudinal study

•Young adults who had experienced ACEs were at higher risk of illicit and problematic drug use.•Cumulative exposure to ACEs increased the likelihood of drug use including problematic drug use in early adulthood.•Substance use prevention programmes should target children exposed to ACEs.•Identified mediators such as interpersonal relationships and school engagement may help guide selection of prevention interventions. Research has demonstrated positive associations between adverse childhood experiences (ACEs) and substance use behaviours in young adults. However, many of these are based on cross-sectional data. The pathway between ACEs and substance misuse among emerging adults is also not fully understood and few studies have investigated potential mediating factors. We analysed data from the Child Cohort of the Growing Up in Ireland study, a nationally representative longitudinal sample of children recruited at 9 years of age (n = 4,729). Logistic regression was used to examine individual and cumulative ACE exposure relationships with substance use at age 20 years. Mediation analyses were used to assess whether parental and peer relationships and school engagement mediate ACE history associations with substance use behaviours. ACE exposure relationships with current cannabis use, other illicit drug use and problematic drug use were observed. In particular, young adults who had experienced 2 ACEs or 3 + ACEs had an approximate two-fold (OR=1.80, 95 % CI: 1.29, 2.51) and approximate three-fold (OR=2.94, 95 % CI: 2.06, 4.19) increased odds of problematic drug use, respectively, when compared to participants who had experienced no ACEs during the study period. Mediation analyses suggested that parent and peer attachment and liking school partially mediate relationships between ACEs and substance use behaviours. Children who have experienced ACEs are at higher risk of future substance use and problematic drug use. In supporting these children, identified mediators such as interpersonal relationships and school engagement may help guide selection of prevention interventions.