Rifaximin Treatment in Bloating Predominant Functional Bowel Disorders

Inclusion Criteria: * Adults aged 18 to 80 years. * Rome IV diagnosis of irritable bowel syndrome, functional constipation, or functional abdominal bloating/distension. * Bothersome bloating with baseline severity of at least 3 on a 7-point Likert scale after adequate constipation treatment, defined as stool frequency from at least 3 times/week to 3 times/day and Bristol Stool Form Scale type 3-5. * Colonoscopy, CT colonography, or barium enema performed if clinically indicated as part of standard evaluation for bowel symptoms. Exclusion Criteria: * History of major gastrointestinal surgery, except appendectomy or laparoscopic cholecystectomy. * Inflammatory bowel disease or other inflammatory gastrointestinal conditions. * Current use of, or inability to discontinue, medications that may affect intestinal microbiota or gas measurements, including antibiotics, proton pump inhibitors, probiotics, lactulose, NSAIDs, or metformin. * Current use of, or inability to discontinue, medications that may affect bloating symptoms, including simethicone, simethicone-containing antispasmodics, or antidiarrheal medications. * Underlying conditions known to affect intestinal microbiota composition, including cirrhosis, uncontrolled diabetes mellitus, end-stage renal disease, obesity, malignancy, or psychiatric disorders. * Opioid-induced constipation. * Known allergy to rifaximin.