Probiotics for major depressive disorder: a systematic review and meta-analysis

This systematic review and meta-analysis examined the effects of probiotics on major depressive disorder (MDD). MEDLINE, EMBASE, PsycINFO, Web of Science, Cochrane Library, ProQuest Dissertations and Theses A&I, and Google Scholar were searched for randomised controlled trials published from inception to 1 March 2025 that compared changes in validated depression rating scale scores among patients diagnosed with MDD who received either probiotics or placebo. Standardised mean differences (SMDs) were calculated using random-effects models; subgroup analyses and meta-regression were performed to identify potential moderators of heterogeneity. In total, 13 studies involving 710 patients with MDD were included in the analysis. The pooled SMD was 0.38, favouring probiotics use. Between-study heterogeneity was moderate, with an I² of 42.8%. Subgroup analyses and meta-regression identified no significant moderators of the SMD. Probiotics appear to reduce depressive symptoms, but factors contributing to heterogeneity remain uncertain.