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By Hans P. Schlecht, MD, MSc, Assistant Professor of Medicine, Department of Medicine, Division of Infectious Diseases & HIV Medicine, Drexel University College of Medicine
Christopher Bruno, MD, Assistant Professor of Medicine, Division of infectious Diseases & HIV Medicine, Drexel University College of Medicine

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Patient Education

Metronidazole is bactericidal. It enters bacterial cell walls and disrupts DNA and inhibits DNA synthesis in certain microorganisms.


Oral metronidazole is absorbed well. It is usually given IV only if patients cannot be treated orally. It is distributed widely in body fluids and penetrates into CSF, resulting in high concentrations.

Metronidazole is metabolized presumably in the liver and excreted mainly in urine, but elimination is not decreased in patients with renal insufficiency.


Metronidazole is active against

  • All obligate anaerobic bacteria (it is inactive against facultative anaerobic and aerobic bacteria)

  • Certain protozoan parasites (eg, Trichomonas vaginalis, Entamoeba histolytica, Giardia intestinalis [lamblia])

Metronidazole is used primarily for infections caused by obligate anaerobes, often with other antimicrobials. Metronidazole is the drug of choice for bacterial vaginosis. The drug has other clinical uses (see Table: Some Clinical Uses of Metronidazole).

Some Clinical Uses of Metronidazole



Infections due to obligate anaerobes (eg, intra-abdominal, pelvic, soft-tissue, periodontal, and odontogenic infections; lung abscess)

Often used with other antimicrobials

Bacterial vaginosis

Drug of choice

Crohn disease

CNS infections (meningitis, brain abscess)



Prophylaxis before intestinal surgery

Clostridium difficile–induced diarrhea (pseudomembranous colitis)

Oral use preferable

Peptic ulcers due to Helicobacter pylori

For treatment and prevention of relapses

Used with other drugs

Acne rosacea

Topical or oral use


Metronidazole is contraindicated in patients who have had an allergic reaction to it.

Use During Pregnancy and Breastfeeding

Metronidazole is in pregnancy category B (animal studies show no risk and human evidence is incomplete, or animal studies show risk but human studies do not). Nonetheless, metronidazole should be avoided during the 1st trimester because mutagenicity is a concern.

Metronidazole enters breast milk; use during breastfeeding is not recommended.

Adverse Effects

Adverse effects include

  • GI disturbances

  • CNS effects and peripheral neuropathy

  • Disulfiram-like reaction

Nausea, vomiting, headache, seizures, syncope, other CNS effects, and peripheral neuropathy can occur; rash, fever, and reversible neutropenia have been reported. Metronidazole can cause a metallic taste and dark urine. A disulfiram-like reaction may occur if alcohol is ingested within 7 days of use.

Dosing Considerations

Metronidazole doses are not decreased in patients with renal failure but are usually decreased 50% in patients with significant liver disease.

Metronidazole inhibits metabolism of warfarin and may increase its anticoagulant effect.

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