Irritable Bowel Syndrome(IBS) is a chronic relapsing disorder with unknown etiology characterized by abdominal pain and bowel habit changes without an organic pathology. Pathophysiology is not fully understood, bacterial overgrowth is one of the reasons. Because of that, use of drugs effective on the gut flora has been raised; Rifaximin is one of them. This study investigated the effectiveness of Rifaximin on IBS.