Treatment of double depression: A meta-analysis

•The treatment literature of double depression (DD) is sparse.•Samples of patients with depression often include patients with DD.•Effect size is large for antidepressants in reducing depressive symptoms.•Treatment effect is moderated by the presence of DD.•A higher percentage of DD patients in a sample is linked to smaller effect size. Double depression (DD), the co-existence of DSM-IV major depressive disorder (MDD) and dysthymia, is a poorly known and sparsely studied phenomenon. Nevertheless, it is prevalent in clinical samples of patients with depression. Thus, it is important to understand the efficacy of its treatment. We conducted a meta-analysis of studies in which antidepressant medication was used to treat depression. Systematic searches in bibliographical databases resulted in 11 samples, including 775 patients that met inclusion criteria. The overall effect size indicating the differences in depressive symptoms before and after pharmacotherapy was 1.81 (95% CI: 1.47, 2.16), suggesting that individuals with depression exhibited a significant reduction in their depressive symptoms following treatment. Importantly, a moderation analysis indicated that a higher proportion of individuals with DD within a sample was associated with lower effect sizes. Publication bias did not pose a major threat to the stability of the findings. High observed heterogeneity indicated substantial variability in effect sizes and elucidation of the potential moderators of treatment outcome was limited due to a paucity of relevant data. Pharmacotherapy seems to be effective in treating DD, but DD may be more difficult to treat than either MDD or dysthymia alone. More research specifically focusing on the treatment of DD with larger sample sizes using randomized control trials is needed to make a firm conclusion.