Metronidazole is a narrow spectrum antibiotic with undoubted efficacy against common anaerobic bacteria; resistance is unusual. Therapeutic concentrations of the drug are attained throughout most body compartments after either oral or intravenous administration. The limited side effects of metronidazole are generally tolerable, transient, or reversible. Clinically, metronidazole is as effective as clindamycin and probably chloramphenicol against anaerobes. It has a definite advantage over clindamycin in CNS infections since clindamycin does not penetrate the CSF well. Metronidazole has no irreversible hematologic toxicities, nor has pseudomembranous colitis been definitely attributed to intravenous use of the drug.