Section 15 · Monograph
Metronidazole
Reviewed Nov 2026FDA label
FlagylNitroimidazole antibiotic
- Forms
- IV: 500mg/100mL | Tab: 250, 500mg
- Dose
- Brain abscess (anaerobic coverage): 500mg IV/PO every 8 hours, usually with a third-generation cephalosporin
- MOA
- Reduced to radicals that damage anaerobe DNA; excellent CNS penetration
Black Box Warning
- Carcinogenic in mice and rats; unnecessary use should be avoided and use reserved for the approved indications
Indications
- Anaerobic bacterial infections including intra-abdominal, CNS (brain abscess, meningitis), bone and joint infectionsFDA-labeled
- Trichomoniasis, amebiasis, bacterial vaginosisFDA-labeled
Mechanism of Action
- Prodrug reduced in anaerobes to radicals that damage DNA; penetrates abscesses and CSF well
Full Dosing
Adult / usual
- Brain abscess (adjunct): 500mg IV or PO every 8 hours, usually combined with a third-generation cephalosporin; oral bioavailability is nearly complete
Hepatic
- Typical duration 6-8 weeks for abscess (guided by imaging); reduce dose in severe hepatic impairment
Contraindications
- Hypersensitivity to nitroimidazoles
- Disulfiram use within the last 2 weeks; alcohol during and for at least 3 days after therapy
- Cockayne syndrome (severe hepatotoxicity)
Drug Interactions
- Warfarin: increased INR
- Lithium levels may rise
- Busulfan, phenytoin and phenobarbital levels altered; alcohol and disulfiram reactions
- QT-prolonging drugs
Adverse Effects
- Peripheral neuropathy and encephalopathy (cumulative dose related)
- Cerebellar toxicity with MRI dentate nucleus changes
- Seizures
- Metallic taste, nausea
- QT prolongation
Monitoring Parameters
- Neurologic symptoms with courses longer than 4 weeks
- CBC with prolonged therapy
- Liver function in hepatic disease
Pregnancy & Lactation
- Used after the first trimester when needed; avoid unnecessary first-trimester use
- Present in milk; consider holding feeds 12-24 hours after a single high dose