ClinCalc Pro
Menu
Calculators Drugs Pathways AI Dx Donate Get the App
Nitroimidazole antibiotic / antiprotozoal Pregnancy: Use with caution in first trimester; widely used in second/third trimester for bacterial vaginosis and trichomoniasis. Single-dose regimen preferred.

Metronidazole

Brand names: Flagyl, Metronidazole

Adult dose

Dose: Anaerobic infections: 400 mg TDS oral or 500 mg TDS IV; C. difficile (mild-moderate): 400–500 mg TDS for 10 days
Route: Oral, IV, or PR (suppository)
Frequency: Three times daily
Max: 4 g/day
Anaerobic prophylaxis (surgical): 500 mg IV at induction. Bacterial vaginosis: 400 mg BD for 5–7 days oral or 0.75% gel intravaginally. Trichomoniasis: 2 g single dose or 400 mg BD for 5–7 days. Giardiasis: 400 mg TDS for 5 days. C. difficile mild: 400–500 mg TDS for 10 days (vancomycin oral preferred for severe/recurrent).

Paediatric dose

Dose: 7.5 mg/kg
Route: Oral or IV
Frequency: Three times daily
Max: 400 mg/dose
Concentration: 5 mg/ml
1 month–11 years: 7.5 mg/kg TDS. ≥12 years: adult dose. C. difficile: 7.5 mg/kg TDS for 10 days. Giardiasis: 2 mg/kg (max 400 mg) TDS for 5 days. Surgical prophylaxis: 7.5 mg/kg IV at induction.

Dose adjustments

Renal

No dose adjustment required for short courses. Prolonged use in severe renal failure — monitor for accumulation.

Hepatic

Reduce dose in severe hepatic impairment; accumulation of active metabolites.

Paediatric weight-based calculator

1 month–11 years: 7.5 mg/kg TDS. ≥12 years: adult dose. C. difficile: 7.5 mg/kg TDS for 10 days. Giardiasis: 2 mg/kg (max 400 mg) TDS for 5 days. Surgical prophylaxis: 7.5 mg/kg IV at induction.

Clinical pearls

  • Excellent anaerobic coverage — cornerstone of surgical prophylaxis combined with cephalosporin
  • Avoid alcohol during treatment AND 48 hours after last dose
  • For severe/recurrent C. difficile: vancomycin oral 125 mg QDS is superior
  • Peripheral neuropathy with prolonged use (>10 days) — monitor and stop if symptoms
  • IV and oral preparations have equivalent bioavailability — switch to oral when tolerated

Contraindications

  • Hypersensitivity to metronidazole or other nitroimidazoles

Side effects

  • Nausea, metallic taste
  • Peripheral neuropathy (prolonged use)
  • Disulfiram-like reaction with alcohol
  • Seizures (high doses or CNS disease)
  • Dark urine (harmless)
  • Elevated LFTs

Interactions

  • Alcohol — disulfiram-like reaction (nausea, vomiting, flushing) — avoid alcohol during treatment and 48 hours after
  • Warfarin — significantly increases INR (reduce warfarin dose, monitor closely)
  • Lithium — can increase lithium levels
  • Phenytoin — increases phenytoin levels
  • Disulfiram — avoid concomitant use (psychosis risk)

Monitoring

  • LFTs (prolonged use)
  • Signs of neuropathy
  • Drug interactions (especially warfarin)

Reference: BNFc; BNF; PHE guidelines; ECDC C. difficile guidelines. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

Calculators
Drugs
Pathways