Persistent gastrointestinal (GI) symptoms after SARS-CoV-2 infection include diarrhea, abdominal pain, dyspepsia, constipation, nausea, and other disorders of gut-brain interaction (DGBIs), but these entities should not be treated as interchangeable. In a meta-analysis of long-COVID studies, any GI symptom was reported in approximately 22% of patients and diarrhea in 10%, although heterogeneity was very high. In a large US Veterans Affairs cohort, post-acute COVID-19 was associated with incident diarrhea (hazard ratio {HR}: 1.58, 95% CI: 1.52-1.65; excess burden 7.99 per 1,000 persons at one year) and irritable bowel syndrome (HR: 1.54, 95% CI: 1.28-1.86). Prospective Rome IV cohorts report lower absolute rates of formally diagnosed DGBIs; one study found persistent DGBI symptoms in 6.6% at six months, with functional diarrhea in 2.2% and irritable bowel syndrome (IBS) in 2.5% at three months. Mechanistic evidence supports intestinal involvement during acute infection and demonstrates associations with epithelial/angiotensin-converting enzyme 2 (ACE2) perturbation, immune activation, barrier dysfunction, altered microbial taxa, and impaired microbial metabolite production. However, persistent viral RNA or antigen, dysbiosis, and gut-lung signaling remain proposed or associative mechanisms rather than established causes of chronic diarrhea. Treatment evidence specific to post-COVID functional diarrhea is sparse. A large synbiotic trial improved a broad "gastrointestinal upset" domain in post-acute COVID syndrome, but did not enroll a defined chronic diarrhea or Rome-positive population; probiotic and fecal microbiota transplantation trials showing GI effects were largely conducted during acute COVID-19. Therefore, current care should first exclude organic and medication-related causes, then apply phenotype-based management extrapolated from established diarrhea-predominant irritable bowel syndrome (IBS-D) and functional-diarrhea practice while clearly acknowledging the limited post-COVID-specific evidence. Standardized definitions, pre-infection GI data, appropriate controls, functional microbiome measures, and dedicated randomized trials are priorities for future research.
