Rifaximin 200-mg tablets are used to treat traveler's diarrhea caused by certain bacteria in adults and children at least 12 years of age. Rifaximin 550-mg tablets are used to prevent episodes of hepatic encephalopathy (changes in thinking, behavior, and personality caused by a build-up of toxins in the brain in people who have liver disease) in adults who have liver disease and to treat irritable bowel syndrome (with diarrhea) in adults. Rifaximin is in a class of medications called antibiotics. Rifaximin treats traveler's diarrhea and irritable bowel syndrome by stopping the growth of the bacteria that cause diarrhea. Rifaximin treats hepatic encephalopathy by stopping the growth of bacteria that produce toxins and that may worsen liver disease. Rifaximin will not work to treat traveler's diarrhea that is bloody or occurs with fever. Antibiotics such as rifaximin will not work for colds, flu, or other viral infections. Taking antibiotics when they are not needed increases your risk of getting an infection later that resists antibiotic treatment.