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Nitroimidazole — Anaerobic Infections / C. difficile / Giardia / Dental Infections Pregnancy: Avoid in first trimester — limited teratogenicity data; widely used in 2nd/3rd trimester for anaerobic infections and trichomonas; considered safe after 14 weeks

Metronidazole (Paediatric)

Brand names: Flagyl, Metronidazole Suspension

Adult dose

Dose: Anaerobic infection: 400 mg every 8 hours orally or 500 mg IV every 8 hours; C. difficile: 400 mg every 8 hours × 10–14 days
Route: Oral, IV, or rectal
Frequency: Every 8 hours
Max: 4.2 g/day IV
Adult reference — see paediatric dose section

Paediatric dose

Dose: 7.5 mg/kg every 8 hours (oral or IV) mg/kg
Route: Oral (suspension 200 mg/5 mL), IV infusion, rectal suppository
Frequency: Every 8 hours
Max: 400 mg per dose; max 1.2 g/day in children
BNFc: neonates — 7.5 mg/kg every 24 hours (term neonates, first 28 days) — slower clearance; preterm — 7.5 mg/kg every 48 hours. Suspension 200 mg/5 mL available. Intra-abdominal sepsis: metronidazole + cefuroxime/ceftriaxone is standard combination. C. difficile in children: oral vancomycin now preferred over metronidazole (IDSA/SHEA 2021); metronidazole reserved for mild cases where vancomycin not available.

Dose adjustments

Renal

No dose adjustment required for renal impairment

Hepatic

Severe hepatic impairment: reduce dose by 50%; extend interval

Paediatric weight-based calculator

BNFc: neonates — 7.5 mg/kg every 24 hours (term neonates, first 28 days) — slower clearance; preterm — 7.5 mg/kg every 48 hours. Suspension 200 mg/5 mL available. Intra-abdominal sepsis: metronidazole + cefuroxime/ceftriaxone is standard combination. C. difficile in children: oral vancomycin now preferred over metronidazole (IDSA/SHEA 2021); metronidazole reserved for mild cases where vancomycin not available.

Clinical pearls

  • Alcohol interaction: disulfiram-like reaction — counsel parents carefully; alcohol-containing medicines (e.g., some cough syrups) should also be avoided during treatment
  • Peripheral neuropathy with prolonged use (>10 days) — tingling and paraesthesia; reverse on stopping; avoid prolonged courses unless necessary (e.g., H. pylori eradication — triple therapy)
  • Metallic taste is prominent — affects compliance; warn in advance; can mask pleasant flavours of suspension
  • Dental: metronidazole first-line for dental abscess (penicillin V + metronidazole covers mixed aerobic/anaerobic oral flora); good gingival/dental tissue penetration

Contraindications

  • First trimester of pregnancy (relative)
  • Hypersensitivity to nitroimidazoles
  • Disulfiram within 2 weeks

Side effects

  • Metallic taste (very common)
  • Nausea and GI disturbance
  • Peripheral neuropathy (prolonged courses)
  • CNS effects (dizziness, headache, seizures — high doses)
  • Disulfiram-like reaction with alcohol
  • Neutropenia (prolonged courses)

Interactions

  • Alcohol — absolute contraindication (disulfiram-like reaction: flushing, vomiting, tachycardia)
  • Warfarin — enhanced INR (CYP2C9 inhibition)
  • Phenytoin — increased phenytoin levels
  • Ciclosporin — increased ciclosporin levels
  • Disulfiram — psychotic reactions

Monitoring

  • Neurological symptoms (peripheral neuropathy — numbness, tingling)
  • LFTs (hepatic impairment dose adjustment)
  • FBC (prolonged courses — neutropenia)

Reference: BNF for Children; NICE NG199 (C. difficile); PHE Antibiotic Guidelines; IDSA/SHEA C. difficile Guidelines 2021. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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