Major depressive disorder is a common comorbidity in persons with substance abuse. The definition of primary major depressive disorder includes symptoms that precede the onset of substance abuse or that are present during periods of abstinence. Substance-induced major depression is indicated when symptoms occur during periods of substance use and exceed what would be expected during intoxication or withdrawal. Various studies have investigated the impact of major depression on relapse rates, but these studies did not differentiate between primary and substance-induced major depression. Hasin and colleagues investigated the effects of major depressive disorder on the outcome of substance dependence in a prospective study.

The study population was recruited from patients in an inpatient substance abuse unit. Those who met the Diagnostic and Statistical Manual of Mental Disorders, 4^th ed. (DSM-IV) diagnosis for cocaine, heroin, or alcohol dependence were followed up at six, 12, and 18 months. The Psychiatric Research Interview for Substance and Mental Disorders was used to assess the participants. Stable remissions from substance abuse (those lasting at least 26 weeks) and relapses of dependence and use were studied.

A total of 250 patients participated in the study. Patients with a history of substance-induced major depression were less likely to have a remission than those who never had major depression. Those who developed major depressive disorder during abstinence had an increased rate of dependence relapse and substance use compared with those who did not have major depressive disorder during abstinence.

The authors conclude that the timing of major depressive disorder in relation to substance dependence can have a substantial impact on remission and relapse rates. Treatment of patients with a history of substance abuse should include an evaluation for major depressive disorder and determination of whether the onset occurred before substance abuse began, during the course of substance abuse, or during abstinence. Further studies are needed to address various treatment plans based on this information.