Weight Gain During SSRI Therapy: The Potential Role of the Gut Microbiota—A Narrative Review and Mechanistic Hypothesis

Introduction: Selective serotonin reuptake inhibitors (SSRIs) remain the first-line pharmacological treatment for depressive disorders. However, long-term treatment is often accompanied by weight gain, which negatively impacts medication adherence and cardiometabolic health. Emerging evidence suggests that, in addition to their effects on the central nervous system (CNS), SSRIs may alter the composition and metabolic activity of the gut microbiome, potentially contributing to metabolic disorders. However, the mechanisms linking SSRI-induced microbial changes to weight regulation remain poorly understood. Methods: This narrative review summarizes the current evidence regarding the interactions between SSRIs, the gut microbiome, host metabolism, and weight regulation. Experimental, translational, and clinical studies were critically assessed, with particular emphasis on recent systematic reviews, meta-analyses, randomized controlled trials, and mechanistic studies. A conceptual mechanistic framework was developed based on the available evidence. Results: Experimental and clinical studies indicate that SSRIs can modify both the taxonomic composition and functional activity of the gut microbiota. These changes may influence the production of microbiota-derived metabolites, including short-chain fatty acids, bile acids, and tryptophan metabolites, thereby influencing gut barrier integrity, low-grade inflammation, gut hormone secretion, appetite regulation, and energy homeostasis. Current evidence also suggests that dietary interventions—including increased dietary fiber intake, adherence to a Mediterranean diet, and microbiota-targeted strategies using prebiotics, probiotics, synbiotics, and postbiotics—can beneficially modulate these pathways. However, no randomized, controlled trials have specifically assessed whether such interventions prevent SSRI-associated weight gain. Conclusions: Taken together, the available evidence raises the possibility that gut microbiota alterations may contribute to SSRI-associated weight gain, although this hypothesis has not yet been directly tested in studies evaluating the complete mechanistic pathway.