Gender Differences in the Time-Varying Relationships Between Substance Use and Depression during Adolescence and Young Adulthood

Introduction: Substance use, including alcohol use, and depression often co-occur; although many previous studies have examined this association, the relationship between substance use and depression is still poorly understood. The large body of research on this association has yielded mixed findings, suggesting that the relationship is probably complex and heterogeneous. It is likely that the etiologies and associations of substance use and depression vary between individuals, as well as over time within individuals. Little research has examined how the association of substance use and depression changes over the lifespan; this study addresses this gap in the literature.

Methods: This study examines how the association between substance use and depression varies from adolescence into young adulthood (age 12 to 31), using data from the National Longitudinal Study of Adolescent and Adult Health (Add Health). We focus on regular cigarette use (20+ days/month), regular heavy episodic drinking (HED: 1+ episodes of HED/month for the past 12 months), and past month marijuana use. Depressive symptoms were assessed with the CES-D 9. We implemented time-varying effect models (TVEM), an analytic approach that estimates how the association between a predictor (e.g., specific substance use measures) and an outcome (e.g., depression) varies across continuous time (e.g., age from 12-31). Whereas typical regression generates a point estimate for a given coefficient linking a predictor and the outcome, TVEM generates a continuous function that describes how the regression coefficient changes across time. All models were stratified by gender and controlled for race/ethnicity.

 Results: Gender differences were observed in both the trajectories of depressive symptoms and substance use as well as the age-varying relationship between substance use and depressive symptoms. Females consistently reported more depressive symptoms and lower substance use than males. Additionally, a stronger association between substance use behaviors and depressive symptoms was observed among females, particularly for cigarette use. Among females, regular cigarette use is associated with a 1-2 point increase in depressive symptoms across ages 12-31, whereas this association was attenuated in males and was not significant during early adulthood. For both males and females, regular HED is strongly associated with increased depressive symptoms (up to 3 points) during adolescence but not after approximately age 18.

 Conclusions: It has been well-documented that women have higher rates of depression; this study suggests that there is a stronger association between substance use and depression among women than men. Understanding how this association varies across the lifespan and differs by gender has important implications for both substance use and mental health prevention and treatment.