Metronidazole: uses, dosing, side effects & brands

Vandazole (Metronidazole) is a nitroimidazole antimicrobial used to treat Abscess, Amebiasis, Bacteroides Infections, Bacterial Endocarditis.

The FDA classifies metronidazole as a nitroimidazole antimicrobial. Nitroimidazole antibiotics (such as metronidazole) are activated inside oxygen-free bacteria and parasites, where they turn into reactive forms that break and destabilize the microbe's DNA. This kills the germ while largely sparing the body's own cells. If you are checking whether it is safe to combine with something else, the class is what matters — two drugs from the same class usually should not be stacked.

The label states there are no adequate and well-controlled studies in pregnant women, but published case-control, cohort, and two meta-analyses covering more than 5,000 pregnant women (many with first-trimester exposure) mostly did not show an increased risk of congenital anomalies or other adverse fetal outcomes. One study suggested an increased risk of cleft lip (with or without cleft palate) that was not confirmed in later work. Metronidazole crosses the placenta and its effects on human fetal organogenesis are not known; animal reproduction studies at doses similar to the maximum human dose showed no evidence of fetal harm. The label does not establish a clear risk. Anyone who is or may become pregnant should discuss use with their prescriber. This is what metronidazole's FDA label says; whether to use it in pregnancy is your prescriber's decision.

The label warns against alcohol. Drinking alcoholic beverages (or taking products containing propylene glycol) during metronidazole therapy, or for at least 3 days after, can cause a disulfiram-like reaction: abdominal cramps, nausea, vomiting, headaches, and flushing. This is cross-referenced to the CONTRAINDICATIONS section. The label separately notes that psychotic reactions have occurred in patients using metronidazole and disulfiram together, and it should not be given to anyone who has taken disulfiram within the prior two weeks. This is what metronidazole's FDA label says; alcohol advice depends on your dose and health, so confirm with your pharmacist.

The elimination half-life of metronidazole is about 8 hours — that is how long the body takes to clear half of a dose. This is the average for healthy adults with normal kidney and liver function; the half-life is much longer in newborns (roughly 22 to 109 hours, longer the more premature the baby). Metronidazole has an active metabolite (hydroxy-metronidazole) whose levels build up in older adults (40-80% higher) and in severe kidney disease, though the label does not give a separate half-life for it. Half-life is not the same as how long a drug test can detect the drug, and it varies with age, kidney and liver function.

It depends on the medicine. We check it against the FDA labels rather than guessing: our interaction checker searches each drug's own label for the other and quotes what it says, naming the section it came from. Run metronidazole against whatever else you take — and remember that a label not naming a drug is not the same as that combination being safe.

We track 16 metronidazole-containing products in the U.S.: Flagyl, Flagyl ER, Vandazole, Flagyl I.V. Rtu in Plastic Container, Likmez, Metro I.V., Metro I.V. in Plastic Container and Metrocream, and 8 more. They are the same active ingredient; they differ in form, manufacturer, price and FDA recall record.

Across the brands we track, metronidazole is currently marketed as capsule, tablet, extended release, topical, injectable, suspension, solution and tablet, per the FDA's National Drug Code Directory. Each form is dosed differently — follow the label for the exact product you were prescribed.

Yes. Our catalog lists 2 generic metronidazole products alongside the brand versions. A generic has the same active ingredient and must meet the FDA's bioequivalence standard; it usually costs less. Ask your pharmacist which one your plan covers.

Alcohol: the FDA label contraindicates drinking on it — see the alcohol section above for what it actually says. Food: beer, wine, spirits, and other products containing alcohol — see the food section above for what each one does and whether it means avoiding it or just being consistent. Other medicines: we hold source-cited interaction answers for Can you drink alcohol while taking metronidazole (Flagyl)?. This is what the FDA label and our cited sources say — it is not a complete list. Your pharmacist can check your own combination.

The most common side effects of Flagyl (metronidazole) are nausea, a sharp, unpleasant metallic taste, headache, loss of appetite, diarrhea, stomach cramps, dry mouth, and a furry or irritated tongue. MedlinePlus lists nausea, vomiting, diarrhea, stomach cramps, loss of appetite, headache and a "sharp, unpleasant metallic taste" among the drug's expected effects. A minority of people notice darkened or reddish-brown urine; the FDA Flagyl label reports this in roughly 1 in 100,000 patients and states it is "almost certainly a metabolite of metronidazole and seems to have no clinical significance" — so it is harmless and fades once you finish the course.

Do not drink any alcohol while taking Flagyl (metronidazole) and for at least 3 days after your last dose. Metronidazole interferes with how your body breaks down alcohol and can trigger a disulfiram-like reaction. MedlinePlus warns to "not drink alcoholic beverages or take products with alcohol or propylene glycol while taking this medication and for at least 3 days after your final dose," because the combination causes "nausea, vomiting, stomach cramps, headache, sweating, and flushing." The reaction can also include a pounding or racing heartbeat and difficulty breathing. Avoid hidden alcohol too — in cough syrups, some mouthwashes, tonics, and elixirs.

Call your doctor right away or seek urgent care if you develop numbness, tingling, or burning pain in your hands or feet, or any seizure. MedlinePlus lists "numbness, pain, burning, or tingling in your hands or feet" and "seizures" as effects needing immediate medical attention, and the FDA Flagyl label describes these as peripheral neuropathy and convulsive seizures that call for prompt discontinuation under a doctor's direction. Also seek urgent help for a serious allergic reaction (rash, hives, blistering or peeling skin, swelling, trouble breathing), signs of aseptic meningitis (stiff neck, fever, severe headache), or severe, watery or bloody diarrhea with stomach cramps — a possible sign of Clostridioides difficile (C. difficile) infection, a known risk with nearly all antibiotics. Do not stop a prescribed antibiotic on your own; contact your prescriber, who will decide whether to stop it.

Flagyl (metronidazole) carries an FDA boxed warning that reads: "Metronidazole has been shown to be carcinogenic in mice and rats. Unnecessary use of the drug should be avoided. Its use should be reserved for the conditions described in the INDICATIONS AND USAGE section." This is why doctors prescribe it only for confirmed or strongly suspected infections and for short courses — a standard 7-to-10-day treatment is not the unnecessary, prolonged, high-dose animal exposure the warning is based on, and no cause-and-effect link has been established in people. The more practical concern with long or repeated courses is nerve damage (peripheral neuropathy), so tell your doctor if you have taken metronidazole many times before.

Effects such as nausea and the metallic taste often appear within the first day or two of starting Flagyl (metronidazole) and usually ease within a few days after you finish the course; the metallic taste and any urine darkening resolve once the drug clears your system (its half-life is about 8 hours). Take the full course exactly as prescribed even once you feel better — MedlinePlus advises: "Continue to take this medication even if you feel well... If you stop taking this medication too soon or skip doses, your infection may not be completely treated and the bacteria may become resistant to antibiotics." This is not a complete list of side effects; ask your pharmacist or prescriber about anything unexpected or persistent.