The chemistry, mechanism of action, pharmacokinetics, and adverse effects of topically applied metronidazole are reviewed, and the drug's use and efficacy in the treatment of rosacea and other conditions are discussed. Metronidazole is a synthetic, nitroimidazole-derivative antibacterial and antiprotozoal agent. For topical use, metronidazole is available in the United States as an aqueous gel. Polar reduction products of the drug appear to be responsible for its antimicrobial effects, which include disruption of DNA. The mechanism by which metronidazole ameliorates the lesions and erythema of rosacea may be related to anti-inflammatory or immunosuppressive actions of the drug rather than to suppression of skin bacteria. Metronidazole does not seem to be appreciably absorbed after topical application to the skin. Little is known about the distribution and elimination of topically applied metronidazole. Topical metronidazole has been designated an orphan drug by the FDA for the treatment of rosacea. In clinical studies metronidazole 0.75% topical gel or 1% cream resulted in improvement in inflammatory lesions in 68-96% of patients. Like other currently available therapies, metronidazole is only palliative; when the drug is withdrawn, symptoms commonly recur. Topical metronidazole has been used with some success in the treatment of decubitus and other ulcers and in certain dental conditions. The drug seems to have low toxicity and generally is well tolerated when applied topically. The principal adverse effects are local reactions, such as burning and stinging. Topical metronidazole provides another option for the treatment of rosacea.