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Antibiotic (Nitroimidazole) Pregnancy: B (second/third trimester) — avoid first trimester if possible

Metronidazole

Brand names: Flagyl, Metrolyl

Adult dose

Dose: 400–500mg three times daily (oral). IV: 500mg three times daily
Route: Oral, IV, or PR
Frequency: Three times daily
Surgical prophylaxis: 500mg IV at induction. C. difficile: 400mg TDS × 10–14 days (mild, first line). Bacterial vaginosis: 400mg BD × 5–7 days.

Paediatric dose

Dose: 7.5 mg/kg
Route: IV or Oral
Frequency: Three times daily
Max: 500mg per dose
Max 1.5g/day for serious infections.
Paediatric weight-based calculator

Max 1.5g/day for serious infections.

Clinical pearls

  • STRICT alcohol avoidance: disulfiram reaction is severe — counsel every patient
  • Active against anaerobes and protozoa: Trichomonas, Giardia, Entamoeba, Gardnerella
  • Peripheral neuropathy with courses >2 weeks — reassess need
  • Resistance increasingly common in H. pylori — often replaced by clarithromycin regimens

Contraindications

  • First trimester of pregnancy (relative)
  • Hypersensitivity to nitroimidazoles

Side effects

  • Metallic taste
  • Nausea
  • Peripheral neuropathy (prolonged use)
  • Disulfiram-like reaction with alcohol
  • Dark urine

Interactions

  • Alcohol — disulfiram reaction (severe nausea, flushing, tachycardia — avoid during and 48h after treatment)
  • Warfarin — significantly potentiates INR
  • Phenytoin — increased levels
  • Lithium — increased levels

Monitoring

  • Neurological symptoms (prolonged courses)
  • LFTs (severe hepatic disease — prolonged half-life)

Reference: BNFc; PHE Antibiotic Guidelines; BNF. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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