Metronidazole is bactericidal. It enters bacterial cell walls and interrupts DNA.

Pharmacology

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.

Indications

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 14: Bacteria and Antibacterial Drugs: Some Clinical Uses of Metronidazole).

Table 14
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Some Clinical Uses of Metronidazole Indication Comments 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's disease — CNS infections (meningitis, brain abscess) — Endocarditis — Septicemia — 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

Contraindications

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.

Last full review/revision July 2009 by Matthew E. Levison, MD