Metronidazole (INN) ( ) is a nitroimidazole anti-infective medication used mainly in the treatment of infections caused by susceptible organisms, particularly anaerobic bacteria and protozoa. It is marketed by Pfizer under the trade name Flagyl in the US, while Sanofi-Aventis markets metronidazole globally under the same tradename, Flagyl, and also by various generic manufacturers, who sell it at a cheaper price. Metronidazole is also used as a gel preparation in the treatment of the dermatological conditions such as rosacea (Rozex and MetroGel by Galderma) and fungating tumours (Anabact, Cambridge Healthcare Supplies).

Metronidazole is a prodrug. It is converted in anaerobic organisms by the redox enzyme pyruvate-ferredoxin oxidoreductase. The nitro group of metronidazole is chemically reduced by ferredoxin (or a ferredoxin-linked metabolic process) and the products are responsible for disrupting the DNA helical structure, thus inhibiting nucleic acid synthesis.

Metronidazole is selectively taken up by anaerobic bacteria and sensitive protozoal organisms because of the ability of these organisms to reduce metronidazole to its active form intracellularly.