Metronidazole is currently approved for use against Trichomonas infection and amebiasis. It is also effective against giardiasis, "nonspecific vaginitis," and anaerobic infections and bactericidal against almost all obligately anaerobic organisms; Actinomyces, Arachnia, and Propionibacterium are exceptions. Metronidazole diffuses well into all tissues and penetrates the central nervous system well. It normally has only a relatively minor impact on the colonic flora. The drug is well tolerated. Adverse reactions include reversible neutropenia, peripheral neuropathy, and disulfiram-like reaction when taken with alcohol. Of more concern are its mutagenicity and carcinogenicity in some, but not all, animals. These are considered low risks, and follow-up studies of patients treated to date do not reveal an increased incidence of cancer, but physicians and patients must decide whether the benefit from therapy outweighs the potential risk. Metronidazole should only be used for approved indications, except in the research setting, and should not be used prophylactically, although it is effective.