Despite previous studies examining the associations between substance use and depression, gaps in the literature persist. In particular, little is known about how the strength of the associations between substance use behaviors and depression change by age, as most studies of this topic examined only adolescents or only adults. This study was designed to elucidate how the associations between substance use behaviors (daily smoking, marijuana use, and HED) and depressive symptoms change from early adolescence into young adulthood (age 12 to 31), using data from the National Longitudinal Study of Adolescent to Adult Health (Add Health). We define substance use behaviors and depressive symptoms more broadly for two reasons: (1) to assess whether the associations previously observed between substance use disorders and MDD generalize to substance use and depressive symptoms and (2) to facilitate examination of these associations in younger individuals, who may be in earlier stages of substance use and depression onset. We implement time-varying effect modeling (TVEM), an analytic approach that estimates the associations between predictors (e.g., smoking, marijuana use, and HED) and an outcome (e.g., depressive symptoms)
