Background: Depressive disorders place a significant disease burden on individuals as well as on societies. Several web-based interventions for depression have shown to be effective in reducing depressive symptoms. However, it is not known whether web-based interventions, when used as adjunctive treatment tools to regular psychotherapy, have an additional effect compared to regular psychotherapy for depression.

Methods: Adults (N = 98) with a unipolar affective disorder were recruited in routine outpatient psychotherapy practices in Germany from therapists over the course of their initial sessions and randomized within therapists to one of two active treatment conditions: regular psychotherapy or psychotherapy plus a web-based depression program ("deprexis"). Primary outcome was depressive symptoms measured with the Beck Depression Inventory (BDI-II) at 12 weeks. Secondary outcomes were anxiety symptoms, somatic symptoms and quality of life at 12 weeks and six months follow-up. The study also included an assessment of the working alliance after six and 12 weeks.

Results: The combination of psychotherapy with the web-based program was more effective than psychotherapy alone at 12 weeks, with medium between-group effect sizes on primary depressive symptoms (Cohen's d = .51) and small to medium between-group effect sizes on secondary outcomes (Cohen's d = .07-.55). Furthermore, we did not observe negative side effects in the blended format, e.g., a lower working alliance than in psychotherapy alone.

Limitations: The study had a smaller than planned sample size and the dropout rate at follow-up was high.

Conclusions: This study provides first evidence that the use of a web-based program as an adjunctive tool in regular psychotherapy could be a promising option to consider in future treatment for depression.