Metronidazole is a prodrug — it's inactive until it enters an anaerobic bacterium or parasite. Inside the organism, the drug's nitro group gets reduced by electron transport proteins (ferredoxin), creating reactive intermediates that attack and break the organism's DNA strands. Metronidazole is a nitroimidazole antimicrobial that kills anaerobic bacteria and parasites via DNA-damaging free radicals. It treats trichomoniasis, amebiasis, and serious anaerobic infections, and prevents post-colorectal-surgery infection. Learn about side effects, interactions, and indications. Metronidazole is used to treat infections of Giardia in dogs, cats, and other companion animals, but it does not reliably clear infection with this organism and is being supplanted by fenbendazole for this purpose in dogs and cats. [105] Sign up today to receive the latest news and updates from UpToDate. © 2026 UpToDate, Inc. and/or its affiliates. All Rights Reserved. Metronidazole is a prescription antibiotic medication prescribed for the treatment of a variety of parasitic and bacterial infections of the vagina, gynecological area, skin, intra-abdominal cavity, blood, bone, joint, nervous system, and heart. Metronidazole treats a wide range of infections, from vaginal and parasitic to dental and skin conditions. Here’s what it’s used for and how it works. Let’s work through both, plus a third one that matters just as much: does metronidazole treat a “stomach bug”? Short answer: it depends entirely on what kind of stomach bug. Learn about the side effects of metronidazole, from common to rare, for consumers and healthcare professionals. Find patient medical information for Metronidazole (Flagyl, Likmez) on WebMD including its uses, side effects and safety, interactions, pictures, warnings, and user ratings Metronidazole belongs to the class of medicines known as antibiotics. It works by killing bacteria or preventing their growth. However, this medicine will not work for colds, flu, or other virus infections. This medicine is available only with your doctor's prescription.