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Nitroimidazole Antibiotic / Antiprotozoal Pregnancy: Avoid in first trimester — carcinogenicity concerns in animal studies (not confirmed in humans). Can be used in second and third trimester for bacterial vaginosis, trichomoniasis, and severe anaerobic infections when benefit outweighs risk.

Metronidazole

Brand names: Flagyl, Metrolyl (IV)

Adult dose

Dose: Anaerobic bacterial infections: 400mg TDS or 500mg TDS for 7 days. Clostridioides difficile (mild-moderate): 400–500mg TDS orally for 10 days. Helicobacter pylori (as part of regimen): 400mg BD for 7 days. Trichomoniasis: 400mg BD for 5–7 days or 2g single dose. Bacterial vaginosis: 400mg BD for 5–7 days or 2g single dose. IV (severe infections/post-surgical): 500mg IV TDS.
Route: Oral / IV / Vaginal
Frequency: Three times daily (most indications)
Max: 4g daily (severe anaerobic infections — IV)
Excellent anaerobic and protozoal cover. Alcohol interaction causes a disulfiram-like reaction — advise strict alcohol avoidance during and for 48 hours after treatment. Metallic taste very common — warn patients. Can turn urine dark brown (harmless — drug metabolite). Avoid prolonged courses (>10 days) — peripheral neuropathy risk.

Paediatric dose

Dose: 7.5 mg/kg
Route: Oral / IV
Frequency: Three times daily
Max: 400mg per dose
BNFc: 1 month–17 years: 7.5mg/kg TDS (max 400mg TDS). Neonates: 7.5mg/kg BD. Seek paediatric specialist opinion for C. difficile in children.

Dose adjustments

Renal

eGFR <10: reduce dose or extend interval — metabolites accumulate (possible CNS toxicity).

Hepatic

Severe hepatic impairment: reduce dose by 50% — extensively hepatically metabolised; accumulation risk and hepatic encephalopathy risk.

Paediatric weight-based calculator

BNFc: 1 month–17 years: 7.5mg/kg TDS (max 400mg TDS). Neonates: 7.5mg/kg BD. Seek paediatric specialist opinion for C. difficile in children.

Clinical pearls

  • Disulfiram-like reaction with alcohol: counsel ALL patients — flushing, palpitations, nausea, hypotension; avoid alcohol during and 48h after completing metronidazole
  • C. difficile: NICE NG199 updated guidance — metronidazole now second-line to fidaxomicin/vancomycin for C. diff (historical first-line status changed); use metronidazole only if others not available/tolerated
  • Peripheral neuropathy risk: limit courses to ≤10 days where possible; if longer treatment needed (e.g. C. difficile recurrence, amoebic liver abscess) — monitor for tingling, numbness in extremities
  • Urine discolouration: warn patients their urine may turn dark brown — harmless metabolite; important as patients may stop medication thinking something is wrong

Contraindications

  • Hypersensitivity to nitroimidazoles
  • First trimester of pregnancy (relative)
  • Alcohol during treatment (disulfiram-like reaction)

Side effects

  • Metallic or bitter taste (very common)
  • Nausea, GI upset
  • Dark urine (harmless)
  • Disulfiram-like reaction with alcohol (flushing, nausea, tachycardia)
  • Peripheral neuropathy (prolonged courses)
  • CNS effects (dizziness, headache; rare — encephalopathy, seizures at high doses)
  • Clostridioides difficile-associated diarrhoea (antibiotic use — ironic, as metronidazole also treats it)

Interactions

  • Alcohol — disulfiram-like reaction; strict avoidance for 48h after last dose
  • Warfarin — potentiates anticoagulant effect; monitor INR closely
  • Lithium — metronidazole increases lithium toxicity risk; monitor levels
  • Phenytoin — increases phenytoin levels

Monitoring

  • Symptom resolution (48–72h for most infections)
  • Liver function (prolonged use)
  • Neurological symptoms (peripheral neuropathy — prolonged use)
  • Alcohol counselling adherence

Reference: BNFc; BNF 90; NICE NG199 (C. difficile); BASHH Bacterial Vaginosis Guidelines. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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