Flagyl (Metronidazole)
Flagyl is the original brand name for metronidazole, a nitroimidazole antimicrobial active against anaerobic bacteria and protozoa. Used for trichomoniasis, bacterial vaginosis, giardiasis, amoebiasis, C. difficile infection and anaerobic infections. Tablets in 200mg, 400mg and 500mg.
- Active ingredients
- Metronidazole
- Manufacturer
- Pfizer
- Dosage forms
- tablet
- Category
- Antibiotics
What is it?
Flagyl is the original brand name for metronidazole, an oral nitroimidazole antimicrobial first introduced in 1960 and approved by the FDA in 1963. In the United States the brand is held by Pfizer, which inherited it through Searle and Pharmacia; in most of Europe Flagyl is marketed by Sanofi. It is one of the few antibiotics that works equally well against anaerobic bacteria and protozoan parasites, which explains its long list of indications. Generic metronidazole has been available for decades and the molecule is on the WHO Model List of Essential Medicines.
Active ingredients
Each Flagyl tablet contains metronidazole as the active ingredient, supplied as 200mg and 400mg tablets in the United Kingdom and much of Europe, and as 250mg and 500mg tablets in the United States, France and Spain. Metronidazole is a prodrug: inside anaerobic bacteria and protozoa it is reduced by low-redox-potential proteins into short-lived radical intermediates that break DNA strands and halt replication. Aerobic cells cannot perform this reduction, which is why the drug spares most normal oxygen-dependent tissue and gut flora.
Forms and dosages
Flagyl is supplied as film-coated tablets, with other presentations (oral suspension, intravenous infusion, vaginal gel and topical cream) available under the same brand in some countries. Typical adult regimens for anaerobic infections are 400mg or 500mg every 8 to 12 hours for 7 to 10 days. For trichomoniasis the prescribing information allows either a single 2g dose or 400–500mg twice daily for 7 days; bacterial vaginosis uses 400–500mg twice daily for 7 days. Tablets are taken with or after food to limit nausea. Treatment beyond 10 days should be reviewed by a doctor because of the neuropathy risk.
Indications
Flagyl is indicated for Trichomonas vaginalis infection in both partners, bacterial vaginosis, giardiasis, intestinal and hepatic amoebiasis, and anaerobic infections of the abdomen, pelvis, bone, brain and soft tissue. It is used in multi-drug regimens to eradicate Helicobacter pylori and as an option for Clostridioides difficile infection, although current guidelines prefer vancomycin or fidaxomicin for most C. difficile cases. Surgical prophylaxis in colorectal procedures and the treatment of dental and gum infections are further labelled uses. According to the prescribing information, the drug should only be used for confirmed or strongly suspected anaerobic or protozoal disease.
How it works
Metronidazole diffuses freely into bacterial and protozoal cells. In organisms with an anaerobic metabolism, ferredoxin and related electron carriers reduce its nitro group, generating nitroso and hydroxylamine radicals that bind DNA, cause strand breakage and loss of the helical structure, and kill the cell. Because the reduction is coupled to the organism's own energy metabolism, the effect is bactericidal rather than merely inhibitory. Oral bioavailability is close to 100%, the half-life is about 8 hours, and the drug reaches therapeutic levels in abscesses, bile, cerebrospinal fluid and vaginal secretions, which supports its use in deep-seated infections.
Frequently asked questions
Why can't I drink alcohol while taking Flagyl? ▾
Metronidazole can produce a disulfiram-like reaction with alcohol: flushing, headache, nausea, vomiting, abdominal cramps and a racing heart. The reaction has been reported even with small amounts of alcohol, including in cough syrups and mouthwash. According to the prescribing information, alcohol should be avoided during treatment and for at least 48 hours after the last dose. If you drank by mistake and feel unwell, contact a doctor or pharmacist.
What does the metallic taste from Flagyl mean? ▾
A metallic or bitter taste is one of the most common side effects of metronidazole and is not a sign of harm. It is caused by the drug and its metabolites being secreted into saliva. Taking tablets with food, using sugar-free gum or rinsing the mouth can ease it, and the taste disappears within a few days of finishing the course. Dark or reddish-brown urine is another harmless effect worth knowing about.
How quickly does Flagyl work for bacterial vaginosis or trichomoniasis? ▾
Symptoms of bacterial vaginosis usually improve within 2 to 3 days, and trichomoniasis clears in over 90% of patients after a single 2g dose or a 7-day course. According to the prescribing information, the full course should be completed even when symptoms resolve early. For trichomoniasis the sexual partner should be treated at the same time, and intercourse avoided until both have finished treatment, to prevent reinfection.
Can Flagyl cause nerve damage? ▾
Peripheral neuropathy (numbness, tingling or burning in the hands and feet) and, rarely, central effects such as dizziness, ataxia or seizures can occur, almost always with prolonged or repeated high-dose courses. Standard 7 to 10 day courses carry a low risk. According to the prescribing information, the drug should be stopped and a doctor consulted if numbness or tingling appears; most cases improve after discontinuation, although recovery can be slow.
Is Flagyl safe in pregnancy and breastfeeding? ▾
Metronidazole crosses the placenta. Large observational studies have not shown an increase in birth defects, and the drug is widely used for bacterial vaginosis and trichomoniasis in pregnancy, although many prescribers avoid the single 2g dose in the first trimester. It passes into breast milk and can give the milk a bitter taste; some guidelines advise withholding breastfeeding for 12 to 24 hours after a single high dose. Discuss the decision with your doctor or midwife.
Does Flagyl interact with other medicines? ▾
Metronidazole inhibits the metabolism of warfarin and can markedly raise the INR, so anticoagulated patients need closer monitoring. It also increases lithium and busulfan levels, and should not be combined with disulfiram because of the risk of psychotic reactions. Phenobarbital and other enzyme inducers lower metronidazole levels. Tell your doctor about all medicines, including over-the-counter products and supplements, before starting treatment.
Related medications
Sources
This page is compiled from the official prescribing information for Metronidazole. The links below open a search for that active ingredient in each register, so you always reach the current version of the label rather than a cached copy.
The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.