Fibromyalgia (FM) and irritable bowel syndrome (IBS) often occur together. Patients with FM and IBS present similar symptoms, such as pain and fatigue; this leads to a delay in diagnosis and management. This systematic review explored the shared pathophysiology of these conditions in adults, focusing on the roles of immune dysfunction, gut dysbiosis, neurotransmitter imbalances, and disturbances in the gut-brain axis. We searched five databases, PubMed, PubMed Central, Google Scholar, Cochrane, and ScienceDirect, for relevant free full-text English articles from 2015 to 2025. Ten studies were selected after screening, identification, and quality assessment, adhering to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines. Our review found that immune system dysregulation involves mast cells and pro-inflammatory cytokines that damage the gut barrier. The gut microbiome and neurotransmitter levels seem to have a reciprocal influence on each other, and their alteration contributes to pathogenesis, with an increase of certain species showing an association with symptom severity. Serotonin and tryptophan metabolism appear to have a crucial role in pain perception, particularly visceral hypersensitivity. Therapeutic strategies targeting the gut microbiome, such as probiotics and fecal microbiota transplantation, have potential but require further research. Overall, this review identified overlapping mechanisms of FM-IBS comorbidity, which can pave the way to effective and combined treatment approaches. Future research should explore gender distinctions in the mechanisms, medications that act on neurotransmitter receptors (especially serotonergic pathways), and the utility of fecal microbiota transplantation and probiotics.
