Substance type moderates the longitudinal association between depression and substance use from pre-treatment through a 1-year follow-up.

BACKGROUND: Research examining directionality of the relationship between depressive symptoms and substance use following treatment entry is limited. Furthermore, substances differ in their neurobiological effects on mood. The relationship between depression and substance use following treatment entry may be moderated by dependence on specific substances. The study tested (a) lagged effects between depressive symptoms and substance use frequency following substance use treatment entry through a 1-year post-treatment follow-up and (b) if substance dependence type moderates these effects.

METHODS: Participants (N = 263) entering residential treatment were assessed for DSM-IV substance dependence, depressive symptoms (Beck Depression Inventory), and percentage of substance use days at post-treatment, 1-, 3-, 6- and 12-month follow-up assessments (time t0 to t4 ). Linear mixed effects models tested lagged effects between depressive symptoms and substance use frequency and the impact of substance type (i.e., dependence on alcohol, cannabis, opioid, cocaine, hallucinogen/PCP) on this relationship.

RESULTS: After controlling for concurrent effects, substance type moderated each longitudinal relationship. Depressive symptoms significantly predicted substance use frequency at the subsequent follow-up assessment, only among individuals with pre-treatment opioid dependence (B = 5.55, SE = 0.89, z = 6.21, p < 0.01). Substance use frequency significantly predicted depressive symptoms at the subsequent follow-up assessment, but not among individuals with cannabis dependence at pre-treatment (B = 1.01, SE = 0.22, t (524) = 4.49, p < 0.01).

CONCLUSIONS: The directionality of depression-substance use comorbidity may differ based on the substance of dependence at pre-treatment. Opioid users may especially benefit from treating both depression and substance use.