Metronidazole (Flagyl)

  • Generic name: Metronidazole
  • Brand names: Flagyl, Rozex, MetroGel
  • Pharmacologic class: Nitroimidazole Antibiotic, Antiprotozoal

Metronidazole uses

  • Bacterial vaginosis (BV), Gardnerella vaginalis.
  • Protozoal infections: trichomoniasis, amebiasis, and giardiasis (lamblia).
  • Anaerobic intra-abdominal and gynecologic infections.
  • Crohn's disease, ulcerative colitis, and Clostridium difficile associated diarrhea (pseudomembranous colitis).
  • Uncomplicated diverticulitis, which represents a localized infection by Gram-negative rods and anaerobes. Metronidazole is used in combination with ciprofloxacin: 500 mg 2-3 times a day plus ciprofloxacin 500 mg twice a day for at least 7 days1.
  • Rosacea.

Metronidazole contraindications

  • Hypersensitivity to metronidazole or other nitroimidazole derivatives.
  • First trimester of pregnancy.
  • Not recommended for persons with active neurological disorders, blood dyscrasias, breast feeding mothers.

Is Metronidazole safe?


  • Metronidazole may cause peripheral neuropathy, especially when used for prolonged periods or at high doses.
  • Metronidazole has been carcinogenic in mice and rats. Avoid unnecessary use.
  • Metronidazole can increase bleeding in patients receiving warfarin or other coumarin anticoagulants. Use concurrently with extreme caution.
  • Metallic taste: The tablets have very unpleasant taste and often leave awful metallic taste in the mouth and furry tongue.
  • Dark urine: May cause darkening of urine (reddish or brown).

Metronidazole and Pregnancy

Pregnancy category: B (in 2nd and 3rd trimesters). Contraindicated in 1st trimester.

There are theoretical safety concerns, as early animal studies showed metronidazole to be mutagenic and potentially carcinogenic. However, the drug has not been shown in humans to act this way, and several epidemiological studies did not reveal that metronidazole use during pregnancy increases the risk of malformations, stillbirths, or low birthweight infants.

Because the non-teratogenicity of metronidazole is difficult to prove it's generally recommended that metronidazole not be taken during pregnancy and breastfeeding.

Alcohol and Metronidazole

Metronidazole can react with alcohol and produce disulfiram-like reaction (nausea, vomiting, and shortness of breath). The possible mechanism of the interaction between alcohol and metronidazole is due to an accumulation of acetaldehyde (such as occurs in a disulfiram reaction) and the inhibition of other enzymes related to alcohol metabolism.

How long after taking metronidazole can you drink alcohol? 48 hours.

Metronidazole alternatives

  • Tinidazole, a nitroimidazole, is the most similar antibiotic to metronidazole.
  • Vancomycin (Vancocin) is an alternative antibiotic for severe Clostridium difficile associated diarrhea.
  • Clindamycin is an effective alternative to metronidazole for treating women with bacterial vaginosis2. Clindamycin is also a good substitute in patients allergic to metronidazole.
  • Furazolidone, an old and cheap antibiotic, is a proven alternative to metronidazole in triple therapy for Helicobacter pylori.
  • Furazolidone and paromomycin (Humatin) are alternative treatments for trichomoniasis3 in people with absolute contraindications to the nitroimidazole antibiotics and for metronidazole-resistant Trichomonas vaginalis.


  • 1. Moya P, Arroyo A, Perez-Legaz J, Serrano P, Candela F, Soriano-Irigaray L, Calpena R. Applicability, safety and efficiency of outpatient treatment in uncomplicated diverticulitis. Tech Coloproctol. 2012 Aug;16(4):301-7.
  • 2. Greaves WL, Chungafung J, Morris B, Haile A, Townsend JL. Clindamycin versus metronidazole in the treatment of bacterial vaginosis. Obstet Gynecol. 1988 Nov;72(5):799-802.
  • 3. Stover KR, Riche DM, Gandy CL, Henderson H. What would we do without metronidazole? Am J Med Sci. 2012 Apr;343(4):316-9 PubMed

Last updated: February, 2015

Good to know

flagyl 400 mg
  • Metronidazole is a primary treatment for mild-to-moderate C. difficile diarrhea. Vancomycin is superior to metronidazole for treating severe diarrhea.
  • Reasons to choose Metronidazole:
  • - Bacteria killing properties: rapidly kills anaerobes, selective action towards anaerobic bacteria.
  • - Low cost
  • - Low risk of inducing C. difficult colitis