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Systematic (IUPAC) name
Clinical data
Trade names Flagyl, Filmet
AHFS/ monograph
Pregnancy cat. B2 (AU) B (US)
Legal status Prescription Only (S4) (AU) -only (CA) POM (UK) -only (US)
Routes oral, topical, rectal, IV, vaginal
Pharmacokinetic data
Bioavailability 80% (oral), 60-80% (rectal), 20-25% (vaginal)[1][2]
Protein binding 20%[1][2]
Metabolism Hepatic[1][2]
Half-life 8 hours[1][2]
Excretion Urine (77%), faeces (14%)[1][2]
CAS number 443-48-1 YesY
ATC code A01AB17 , D06BX01, G01AF01, J01XD01, P01AB01, QP51AA01
PubChem CID 4173
DrugBank DB00916
ChemSpider 4029 YesY
KEGG D00409 N
NIAID ChemDB 007953
Chemical data
Formula C6H9N3O3 
Mol. mass 171.15 g/mol
Physical data
Melt. point 159–163 °C (318–325 °F)
 N (what is this?)  (verify)

Metronidazole (AAN, BAN, INN, USAN) /mɛtrəˈndəzl/ (Flagyl and others) is a nitroimidazole antibiotic medication used particularly for anaerobic bacteria and protozoa. Metronidazole is an antibacterial against anaerobic organisms, amoebicide and antiprotozoal.[3] It is the drug of choice for first episodes of mild-to-moderate Clostridium difficile infection.[4]

It is on the World Health Organization's List of Essential Medicines, a list of the most important medication needed in a basic health system.[5]

Medical uses[edit]

Metronidazole is primarily used to treat: bacterial vaginosis, pelvic inflammatory disease (along with other antibacterials like ceftriaxone), pseudomembranous colitis, aspiration pneumonia, rosacea (topical), fungating wounds (topical), intra-abdominal infections, lung abscess, gingivitis, amoebiasis, giardiasis, trichomoniasis, and infections caused by susceptible anaerobic organisms such as Bacteroides fragilis, spp, Fusobacterium spp, Clostridium spp, Peptostreptococcus spp and Prevotella spp.[6] It is also often used to eradicate Helicobacter pylori along with other drugs and to prevent infection in people recovering from surgery.[6]

Bacterial vaginosis[edit]

Drugs of choice for the treatment of bacterial vaginosis include metronidazole and clindamycin. The treatment of choice for bacterial vaginosis in non-pregnant women include metronidazole oral twice daily for seven days, or metronidazole gel intravaginally once daily for five days, or clindamycin intravaginally at bedtime for seven days. For pregnant women, the treatment of choice is metronidazole oral three times a day for seven days. Data does not report routine treatment of male sexual partners in non-pregnant or pregnant women.[7]


The 5-nitroimidazole drugs (metronidazole and tinidazole) are the mainstay of treatment for infection with Trichomonas vaginalis. Treatment for both the infected patient and the patient's sexual partner is recommended, even if asymptomatic. Therapy other than 5-nitroimidazole drugs is also an option, but cure rates are much lower.[8]

Clostridium difficile colitis[edit]

Initial antibiotic therapy for non-severe Clostridium difficile colitis (pseudomembranous colitis) consists of oral metronidazole or oral vancomycin. Several randomized controlled trials have demonstrated equivalent efficacy of oral metronidazole and oral vancomycin in treating non-severe Clostridium difficile colitis.[9][10][11] However, it has been shown that oral vancomycin is more effective in treating patients with severe Clostridium difficile colitis.[9]

Entamoeba histolytica[edit]

Invasive colitis and extraintestinal disease including liver abscesses, pleuropulmonary infections, and brain abscesses can result from infection with Entamoeba histolytica. Metronidazole is a widely used drug in patients with invasivs colitis, liver abscesses, pleuropulmonary infection, and/or brain abscesses caused by Entamoeba histolytica.

Preterm births[edit]

Metronidazole has also been used in women to prevent preterm birth associated with bacterial vaginosis, amongst other risk factors including the presence of cervicovaginal fetal fibronectin (fFN). A randomised controlled trial demonstrated that metronidazole was ineffective in preventing preterm delivery in high-risk pregnant women (selected by history and a positive fFN test) and, conversely, the incidence of preterm delivery was found to be higher in women treated with metronidazole.[12]

Adverse effects[edit]

Common adverse drug reactions (≥1% of those treated with the drug) associated with systemic metronidazole therapy include: nausea, diarrhoea, weight loss, abdominal pain, vomiting, headache, dizziness and metallic taste in the mouth. Intravenous administration is commonly associated with thrombophlebitis. Infrequent adverse effects include: hypersensitivity reactions (rash, itch, flushing, fever), headache, dizziness, vomiting, glossitis, stomatitis, dark urine and paraesthesia.[6] High doses and/or long-term systemic treatment with metronidazole is associated with the development of leucopenia, neutropenia, increased risk of peripheral neuropathy and CNS toxicity.[6]

Common adverse drug reaction associated with topical metronidazole therapy include local redness, dryness and skin irritation; and eye watering (if applied near eyes).[6]

Metronidazole may cause mood swings. There is some evidence from studies in rats that supports the possibility of metronidazole may contribute to serotonin syndrome, although no case reports documenting this have been published to date.[13][14]

Mutagenesis and carcinogenesis[edit]

Metronidazole is listed by the US National Toxicology Program (NTP) as reasonably anticipated to be a human carcinogen.[15] Although some of the testing methods have been questioned, oral exposure has been shown to cause cancer in experimental animals and has also demonstrated some mutagenic effects in bacterial cultures.[15][16] The relationship between exposure to metronidazole and human cancer is unclear.[15][17] One study (Beard et al. 1988) found an excess in lung cancer among women (even after adjusting for smoking), while other studies (IARC 1987; Thapa et al. 1998) found either no increased risk, or a statistically insignificant risk.[15] [18] Metronidazole is listed as a possible carcinogen according to the WHO International Agency for Research on Cancer (IARC).[19] A study in those with Crohn's disease also found chromosomal abnormalities in circulating lymphocytes in people treated with metronidazole.[16]

Due to its potential carcinogenic properties, metronidazole is banned in the European Union and the USA for veterinary use in the feed of animals and is banned for use in any food animals in the USA.[20][21]

Interaction with alcohol[edit]

Consuming ethanol (alcohol) while taking metronidazole has long been thought to have a disulfiram-like reaction with effects that can include nausea, vomiting, flushing of the skin, tachycardia (accelerated heart rate), and shortness of breath.[22] It is typically advised that consumption of alcohol should be avoided by patients during systemic metronidazole therapy and for at least 48 hours after completion of treatment.[6] However there are studies calling into question the mechanism of the interaction of alcohol and metronidazole,[23] ,[24][25] and a possible central toxic serotonin reaction for the alcohol intolerance suggested.[13] Metronidazole is also generally thought to inhibit the liver metabolism of propylene glycol (found in some foods, medicines and in many electronic cigarette e-liquids), and thus propylene glycol may potentially have similar interaction effects with metronidazole.[citation needed]

Other drug interactions[edit]

It also inhibits CYP2C9 and CYP3A4 and hence may interact with medications that are metabolised by this enzyme (e.g. lomitapide, warfarin).[1]

Stevens–Johnson syndrome[edit]

Metronidazole alone rarely causes Stevens–Johnson syndrome but is reported to occur at high rates when combined with mebendazole.[26]

Mechanism of action[edit]

It inhibits nucleic acid synthesis by disrupting the DNA of microbial cells.[1] This function only occurs when metronidazole is partially reduced, and because this reduction usually happens only in anaerobic cells, it has relatively little effect upon human cells or aerobic bacteria.[27]


2-Methylimidazole (1) may be prepared via the Debus-Radziszewski imidazole synthesis, or from ethylenediamine and acetic acid, followed by treatment with lime, then Raney nickel. 2-Methylimidazole nitrated to give 2-methyl-4(5)-nitroimidazole (2), which is in turn alkylated with ethylene oxide or 2-chloroethanol to give metronidazole (3):[28][29][30]

Synthesis of metronidazole.png

Veterinary use[edit]

Metronidazole is not labeled for animal use but is widely used to treat infections of Giardia in dogs, cats, and other companion animals, although it does not reliably clear infection with this organism and is being supplanted by Fenbendazole for this purpose in dogs and cats.[31] Metronidazole is also used for the management of chronic inflammatory bowel disease in cats and dogs.[32] Another common usage is the treatment of systemic and/or GI clostridial infections in horses. Metronidazole or simply "Metro" is used in the aquarium hobby to treat ornamental fish and as a wide spectrum treatment for bacterial and protozoan infections in reptiles and amphibians. It is also used to treat human enteric (gi) and systemic infections. In general, the veterinary community may use metronidazole for any potentially susceptible anaerobic infection. The U.S. Food and Drug Administration (FDA) suggests that it only be used when necessary because it has been shown to be carcinogenic in mice and rats as well as the microbes that it is prescribed to fight. The main reason that it should be used sparingly, however, is the fact that it still works, thus it shouldn't be abused lest the microbes develop a resistance.[33][34]


  1. ^ a b c d e f g "Flagyl, Flagyl ER (metronidazole) dosing, indications, interactions, adverse effects, and more". Medscape Reference. WebMD. Retrieved 3 April 2014. 
  2. ^ a b c d e Brayfield, A, ed. (14 January 2014). "Metronidazole". Martindale: The Complete Drug Reference. Pharmaceutical Press. Retrieved 3 April 2014. 
  3. ^ [monograph "Metronidazole monograph"]. Retrieved 3 April 2014. 
  4. ^ Cohen, S. H.; Gerding, D. N.; Johnson, S.; Kelly, C. P.; Loo, V. G.; McDonald, L.  C.; Pepin, J.; Wilcox, M. H.; Society for Healthcare Epidemiology of America; Infectious Diseases Society of America (2010). "Clinical Practice Guidelines for Clostridium difficile Infection in Adults: 2010 Update by the Society for Healthcare Epidemiology of America (SHEA) and the Infectious Diseases Society of America (IDSA)". Infection Control and Hospital Epidemiology 31 (5): 431–455. doi:10.1086/651706. PMID 20307191.  edit
  5. ^ "WHO Model List of EssentialMedicines". World Health Organization. October 2013. Retrieved 22 April 2014. 
  6. ^ a b c d e f Rossi, S, ed. (2013). Australian Medicines Handbook (2013 ed.). Adelaide: The Australian Medicines Handbook Unit Trust. ISBN 978-0-9805790-9-3.  edit
  7. ^ Joesoef, MR; Schmid, GP; Hillier, SL (Jan 1999). "Bacterial vaginosis: review of treatment options and potential clinical indications for therapy.". Clinical infectious diseases : an official publication of the Infectious Diseases Society of America. 28 Suppl 1: S57–65. PMID 10028110. 
  8. ^ Dubouchet, L; Spence, M. R.; Rein, M. F.; Danzig, M. R.; McCormack, W. M. (1997). "Multicenter comparison of clotrimazole vaginal tablets, oral metronidazole, and vaginal suppositories containing sulfanilamide, aminacrine hydrochloride, and allantoin in the treatment of symptomatic trichomoniasis". Sexually transmitted diseases 24 (3): 156–60. PMID 9132982.  edit
  9. ^ a b Zar, F. A.; Bakkanagari, S. R.; Moorthi, K. M.; Davis, M. B. (2007). "A comparison of vancomycin and metronidazole for the treatment of Clostridium difficile-associated diarrhea, stratified by disease severity". Clinical Infectious Diseases 45 (3): 302–7. doi:10.1086/519265. PMID 17599306.  edit
  10. ^ Wenisch, C; Parschalk, B; Hasenhündl, M; Hirschl, A. M.; Graninger, W (1996). "Comparison of vancomycin, teicoplanin, metronidazole, and fusidic acid for the treatment of Clostridium difficile-associated diarrhea". Clinical infectious diseases : an official publication of the Infectious Diseases Society of America 22 (5): 813–8. PMID 8722937.  edit
  11. ^ Teasley, D. G.; Gerding, D. N.; Olson, M. M.; Peterson, L. R.; Gebhard, R. L.; Schwartz, M. J.; Lee Jr, J. T. (1983). "Prospective randomised trial of metronidazole versus vancomycin for Clostridium-difficile-associated diarrhoea and colitis". Lancet 2 (8358): 1043–6. PMID 6138597.  edit
  12. ^ Shennan, A.; Crawshaw, S.; Briley, A.; Hawken, J.; Seed, P.; Jones, G.; Poston, L. (2005). "General obstetrics: A randomised controlled trial of metronidazole for the prevention of preterm birth in women positive for cervicovaginal fetal fibronectin: The PREMET Study". BJOG: An International Journal of Obstetrics and Gynaecology 113 (1): 65–74. doi:10.1111/j.1471-0528.2005.00788.x. PMID 16398774.  edit
  13. ^ a b Karamanakos, P.; Pappas, P.; Boumba, V.; Thomas, C.; Malamas, M.; Vougiouklakis, T.; Marselos, M. (2007). "Pharmaceutical Agents Known to Produce Disulfiram-Like Reaction: Effects on Hepatic Ethanol Metabolism and Brain Monoamines". International Journal of Toxicology 26 (5): 423–432. doi:10.1080/10915810701583010. PMID 17963129.  edit
  14. ^ Karamanakos, P. N. (2008). "The possibility of serotonin syndrome brought about by the use of metronidazole". Minerva Anestesiologica 74 (11): 679. PMID 18971895.  edit
  15. ^ a b c d "Metronidazole CAS No. 443-48-1" (pdf). Report on Carcinogens, Twelfth Edition (2011). U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program. Retrieved 2011-10-28. 
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  18. ^ "Flagyl 375 U.S. Prescribing Information" (pdf). Pfizer. 
  19. ^ International Agency for Research on Cancer (IARC) (May 2010). "Agents Classified by the IARC Monographs, Volumes 1–100" (PHP). World Health Organization. Retrieved 2010-06-06. 
  20. ^ Committee for Medicinal Products for Veterinary Use (CVMP) (July 1997). "Metronidazole Summary Report EMEA/MRL/173/96-FINAL" (pdf). European Medicines Agency. Retrieved 2009-12-11. 
  21. ^ "Restricted and Prohibited Drugs in Food Animals". Food Animal Residue Avoidance Databank. U.S. Food and Drug Administration. 2013. Retrieved 2013-12-28. 
  22. ^ Cina, S. J.; Russell, R. A.; Conradi, S. E. (1996). "Sudden death due to metronidazole/ethanol interaction". The American Journal of Forensic Medicine and Pathology 17 (4): 343–346. PMID 8947362.  edit
  23. ^ Gupta, N. K.; Woodley, C. L.; Fried, R. (1970). "Effect of metronidazole on liver alcohol dehydrogenase". Biochemical Pharmacology 19 (10): 2805–2808. doi:10.1016/0006-2952(70)90108-5. PMID 4320226.  edit
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  25. ^ Visapää, J. P.; Tillonen, J. S.; Kaihovaara, P. S.; Salaspuro, M. P. (2002). "Lack of disulfiram-like reaction with metronidazole and ethanol". The Annals of Pharmacotherapy 36 (6): 971–974. PMID 12022894.  edit
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  27. ^ Eisenstein, B. I.; Schaechter, M. (2007). "DNA and Chromosome Mechanics". In Schaechter, M.; Engleberg, N. C.; DiRita, V. J. et al. Schaechter's Mechanisms of Microbial Disease. Hagerstown, MD: Lippincott Williams & Wilkins. p. 28. ISBN 978-0-7817-5342-5. 
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  30. ^ Kraft, M. Ya.; Kochergin, P. M.; Tsyganova, A. M.; Shlikhunova, V. S. (1989). "Synthesis of metronidazole from ethylenediamine". Pharmaceutical Chemistry Journal 23 (10): 861–863. doi:10.1007/BF00764821. 
  31. ^ Barr, S. C.; Bowman, D. D.; Heller, R. L. (1994). "Efficacy of fenbendazole against giardiasis in dogs". American Journal of Veterinary Research 55 (7): 988–990. PMID 7978640.  edit
  32. ^ Hoskins, J. D. (Oct 1, 2001). "Advances in managing inflammatory bowel disease". DVM Newsmagazine. Retrieved 2013-12-28. 
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  34. ^ "Metronidazole". 

External links[edit]

  • "Metronidazole". Drug Information Portal. U.S. National Library of Medicine.