Metronidazole, a nitroimidazole derivative, is a unique antimicrobial agent that is active against both bacterial and parasitic organisms, although only the anaerobic members of these groups are susceptible. It has been used for the treatment of trichomoniasis for about 20 years and is also effective against amebiasis and giardiasis. More recently, metronidazole has emerged as a principal agent for the treatment of anaerobic bacterial infections. It is highly effective against all species of anaerobes except certain non-spore-forming gram-positive bacilli and cocci and is the only agent rapidly bactericidal against the Bacteroides fragilis group. Clinical studies have proved its efficacy in prophylaxis for elective colorectal surgical procedures and in the treatment of deep abdominal sepsis (usually in combination with another agent, such as an aminoglycoside). Metronidazole is the treatment of choice for nonspecific vaginitis that seems to be a mixed infection due to anaerobes and Gardnerella vaginalis. Adequate concentrations in the blood are produced after both oral and intravenous administration, and the side effects are infrequent and minimal.