The co-occurrence of substance use and depression has important implications for the treatment both of substance use and mental health problems, as individuals with co-occurring disorders often have more severe substance use and mental health problems compared to individuals with standalone disorders (Couwenberg et al., 2006; Grella et al., 2001; Rowe et al., 2004). Comorbidity is associated with worse alcohol and drug treatment outcomes among both adults (Brown et al., 2004; Compton et al., 2003; Timko et al., 2010) and adolescents (Cornelius et al., 2004; Vourakis, 2005; White et al., 2004), as well as higher rates of suicide (Davis et al., 2008). Our results demonstrate significant associations between substance use behaviors and depressive symptoms throughout adolescence and young adulthood and allude to a role of polysubstance use in depression. Thus, it is critical that prevention and treatment efforts focus jointly on substance use behaviors and depression among this population. A similar recommendation was made by Brook et al. (2014), who advised that treatment for depression should particularly emphasize tobacco prevention and cessation. The chronicity of the associations between depressive symptoms
