Why is metronidazole used against anaerobic bacteria

MetronidazoleDrug groupsAntibioticsNitroimidazole Metronidazole is an active ingredient from the group of nitroimidazoles, which is used in the form of tablets for the treatment of infections with bacteria and parasites. It has both bactericidal properties against anaerobic bacteria and anti-parasitic effects against protozoa. The indications include bacterial infections, amebiasis, trichomoniasis and giardiasis. The tablets are taken with or after meals. Metronidazole should only be administered for a short time and for a maximum of 10 days. It must not be combined with alcohol, as antabuse effects can occur. Possible adverse effects include central and peripheral disorders, gastrointestinal discomfort, a metallic taste, and rashes. Metronidazole can darken the urine.

synonymous: metronidazolum, MTZ


Metronidazole is available in various dosage forms for systemic and local therapy. This article refers to the film-coated tablets (Flagyl® and generics). The drug has been approved in Switzerland since 1960.

Structure and properties

Metronidazole (C.6H9N3O3, Mr = 171.2 g / mol) is a derivative of imidazole, which is substituted with a nitro group, a methyl group and ethanol. It is available as a white to yellowish, crystalline powder with a bitter taste, sensitive to light and sparingly soluble in water. The active ingredient was developed at Rhône-Poulenc on the basis of azomycin, a natural substance that was derived from a Streptomyces- Kind has been isolated.


Metronidazole (ATC J01XD01) has both bactericidal properties against anaerobic bacteria and anti-parasitic effects against protozoa (single-cell organisms). It is a prodrug that is metabolized under anaerobic conditions in the cell to nitroso radicals, which attack the DNA. Strand breaks, an inhibition of DNA synthesis and cell death occur. Metronidazole is well distributed in the tissues and has a half-life of 8 hours (6 to 10 hours).

  • For the treatment of infections with sensitive anaerobic bacteria.
  • Intestinal and Hepatic Amebiasis (Entamoeba histolytica, Parasite).
  • Trichomoniasis (Trichomonas vaginalis, Parasite).
  • Bacterial vaginosis, Gardnerella vaginalis infections (Gardnerella vaginalis, Bacterium).
  • Lambliasis (giardiasis, Giardia intestinalis, Parasite).

According to the specialist information. The tablets are taken between one to four times a day for several days. Brief therapy can be used for some infections. A high dose is given once or twice.

The German product information recommends taking the tablets during or after meals.

Metronidazole is not intended for long-term therapy. As a rule, the duration of therapy should not exceed 10 days. The treatment should be repeated as rarely as possible. The duration of treatment was limited because damage to human germ cells cannot be ruled out and animal experiments have shown mutagenic and carcinogenic effects.


The complete precautionary measures can be found in the medicinal product information sheet.


Shortly before, during and at least one day after the treatment, alcohol should not be drunk or ingested, as antabuse effects can occur.

These include flushing, nausea, vomiting, a fast heartbeat, headache, abdominal cramps and dizziness. The US-American specialist information even recommends a parental leave of three Days after weaning and also mentions that propylene glycol must also be avoided.

Other drug interactions have been described with vitamin K antagonists, disulfiram, enzyme inducers, enzyme inhibitors, lithium, ciclosporin, 5-fluorouracil and busulfan.

unwanted effects

Possible adverse effects, which are uncommon to common, include:

  • Nausea, upper abdominal pain, vomiting, diarrhea, loss of appetite, glossitis, inflammation of the oral mucosa
  • Taste disorders: metallic taste, bitter eructation
  • Sensory disturbances (paraesthesia)
  • Headache, dizziness, depression Insomnia, weakness, abnormal gait, confusion
  • Rash, itching, flush

Metronidazole can darken the urine. The specialists should inform the patient of this so that there is no uncertainty.

Checklist for advising patients

Download: Checklist_Metronidazole.pdf

  • Pharmaceutical product information (CH, D, USA)
  • Bendesky A., Menéndez D., Ostrosky-Wegman P. Is metronidazole carcinogenic? Mutat Res, 2002, 511 (2), 133-44 Pubmed
  • European Pharmacopoeia PhEur
  • Freeman C.D., Klutman N.E., Lamp K.C. Metronidazoles. A therapeutic review and update. Drugs, 1997, 54 (5), 679-708 Pubmed
  • Kafadar I. et al. A rare adverse effect of metronidazole: nervous system symptoms. Pediatr Emerg Care, 2013, 29 (6), 751-2 Pubmed
  • Löfmark S., Edlund C., Nord C.E. Metronidazole is still the drug of choice for treatment of anaerobic infections. Clin Infect Dis, 2010, 50 Suppl 1, S16-23 Pubmed
  • Samuelson J. Why metronidazole is active against both bacteria and parasites. Antimicrob Agents Chemother, 1999, 43 (7), 1533-41 Pubmed
  • Sneader W. Drug discovery: a history. Chichester: Wiley, 2005
  • Sobel R., Sobel J.D. Metronidazole for the treatment of vaginal infections. Expert Opin Pharmacother, 2015, 16 (7), 1109-15 Pubmed

Conflicts of Interest: None / Independent. The author has no relationships with the manufacturers and is not involved in the sale of the products mentioned.

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