Rifampin is a semisynthetic derivative of rifamycin B that is active against a wide range of microorganisms and bactericidal for extracellular as well as intracellular pathogens. Most gram-positive and many gram-negative bacteria, as well as mycobactenia, are inhibited by nifampin. Staphylococcus aureus, coagulase-negative staphylococci, Neisseria meningitidis, N gonorrheae, and Haemophilus influenzae are very sensitive to rifampin. The drug works by inhibiting DNA-dependent RNA polymerase, thus preventing the initiation of chain formation in RNA synthesis. Rifampin is available in the United States in oral and intravenous preparations. It is almost completely absorbed from the gastrointestinal tract after oral administration, reaching peak plasma concentrations in 2 to 4 hours.