XIFAXAN tablets contain rifaximin, a non-aminoglycoside semi-synthetic, nonsystemic antibiotic derived from rifamycin SV. Rifaximin is a structural analog of rifampin. The chemical name for rifaximin is (2S,16Z,18E,20S,21S,22R,23R,24R,25S,26S,27S,28E)?5,6,21,23,25- pentahydroxy-27-methoxy-2,4,11,16,20,22,24,26-octamethyl-2,7( epoxypentadeca-[1,11,13]trienimino)benzofuro[4,5-e]pyrido[1,2-??]-benzimidazole-1,15(2H)?dione, 25-acetate. The empirical formula is C43H51N3O11 and its molecular weight is 785.9. The chemical structure is represented below: