Rifamycin S is an impurity of Rifaximin. Rifaximin is a semi-synthetic, nonsystemic antibiotic derived from rifamycin SV with antibacterial activity. It also acts as a pregnane X receptor (PXR) agonist. It is approved for the treatment of traveler's diarrhea, irritable bowel syndrome, and hepatic encephalopathy. It also has anti-gram-positive bacterial and mycobacterium activities.