Abstract
Background Comorbid major depression (MD) and alcohol use disorder (AUD), particularly in adolescents, have been shown to be associated with poorer subsequent MD outcomes. Methods Longitudinal data were used to model associations between a four-level classification of MD/AUD during the period 15–18 years (neither; MD-only; AUD-only; comorbid MD/AUD) and MD over the period 18–35 years. These associations were then adjusted for confounding by a series of factors measured in childhood. Results The three disorder groups had rates of adult MD during the period 18–35 years that were significantly (p<.05) higher than that of the group with no disorder. Furthermore, those in the comorbid MD/AUD group had significantly (p <.05) higher rates of adult MD than those in the AUD-only group, and marginally (p <.10) higher rates of adult MD than those in the MD-only group. After adjustment for confounding, the difference in rates of adult MD between the MD-only group and the MD/AUD group were no longer statistically significant. The factors that explained the associations were gender, childhood behavior problems, and exposure to physical and sexual abuse. Limitations The data were obtained by self-report, and may have been subject to biases. Conclusions The results of these analyses suggest that marginally higher rates of depression to age 35 amongst the comorbid MD/AUD group were explained by increased exposure to adverse childhood circumstances amongst members of the comorbid group. Adolescent MD/AUD comorbidity is likely to be a risk marker, rather than a causal factor in subsequent MD.