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Antibiotic / Antiprotozoal Pregnancy: Use if clinically indicated — BASHH 2023 supports treatment of BV in all trimesters; avoid alcohol during and 48h after treatment

Metronidazole

Brand names: Flagyl, Metrogel, Zidoval

Adult dose

Dose: 400–500 mg
Route: Oral / IV
Frequency: Twice daily (oral); every 8 hours (IV for severe infection)
Max: 4 g/24h (IV); 800 mg/day (oral bacterial vaginosis); variable by indication
Used for bacterial vaginosis (BV), Trichomonas vaginalis, pelvic inflammatory disease (PID), post-operative infection prophylaxis. Vaginal gel (0.75%) for BV — Zidoval. Current BASHH 2023 guidance: treat if symptomatic regardless of trimester.

Paediatric dose

Dose: 7.5 mg/kg mg/kg
Route: Oral / IV
Frequency: Every 8 hours
Max: 400 mg per dose
BNFc: neonates — 15 mg/kg loading dose then 7.5 mg/kg every 12 hours

Dose adjustments

Renal

No dose adjustment for mild-moderate renal impairment; reduce dose frequency in severe impairment (eGFR <10)

Hepatic

Reduce dose in severe hepatic impairment — hepatic metabolism; accumulation risk

Paediatric weight-based calculator

BNFc: neonates — 15 mg/kg loading dose then 7.5 mg/kg every 12 hours

Clinical pearls

  • BASHH 2023: treat BV with metronidazole regardless of trimester — untreated BV associated with preterm birth and late miscarriage
  • Historically avoided in first trimester due to theoretical teratogenicity concern — large cohort studies have not confirmed increased risk
  • Counsel all patients to avoid alcohol during treatment and for 48 hours after (disulfiram-like reaction: flushing, vomiting, tachycardia)
  • For PID: combined with doxycycline and ceftriaxone (BASHH PID guidelines 2019) — metronidazole covers anaerobic organisms
  • Vaginal metronidazole gel (Zidoval) for BV — lower systemic absorption than oral; preferred in first trimester by some guidelines
  • Significant warfarin interaction — INR check within 3–5 days of starting treatment

Contraindications

  • Hypersensitivity to nitroimidazoles
  • Disulfiram within 2 weeks (psychotic reactions reported)

Side effects

  • Nausea
  • Metallic taste
  • Peripheral neuropathy (prolonged use)
  • Disulfiram-like reaction with alcohol
  • QT prolongation (high dose IV)

Interactions

  • Alcohol — disulfiram-like reaction; must abstain during and 48h after treatment
  • Warfarin — potentiates anticoagulant effect significantly (INR monitoring essential)
  • Lithium — increased toxicity risk
  • Disulfiram — psychotic reactions

Monitoring

  • Symptom response
  • INR (if on warfarin)
  • Peripheral neurological symptoms (prolonged use)
  • LFTs (severe hepatic impairment)

Reference: BNFc; BNF 90; BNFc; BASHH BV Guidelines (2023); BASHH PID Guidelines (2019); NICE CG190. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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