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Antidote — paracetamol overdose / Hepatoprotectant Pregnancy: Use in pregnancy — fetal liver also protected.

Acetylcysteine (N-acetylcysteine — NAC)

Brand names: Parvolex

Adult dose

Dose: 3-bag regimen: 150 mg/kg in 200 mL 5% glucose over 1h; then 50 mg/kg in 500 mL over 4h; then 100 mg/kg in 1L over 16h
Route: IV infusion
Frequency: Single 21-hour course (3 bags)
Max: 300 mg/kg total over 21 hours
Paracetamol overdose: start as soon as possible if above treatment line on nomogram OR if >150 mg/kg ingested within 24h and cannot plot time. New MHRA-approved modified regimen (2-bag): 200 mg/kg in 200 mL over 4h, then 100 mg/kg in 1L over 16h. Check LFTs, INR, creatinine at end of infusion.

Paediatric dose

Dose: 150 mg/kg
Route: IV
Frequency: 3-bag regimen over 21 hours
Max: 300 mg/kg total
Concentration: 200 mg/ml
3-bag regimen — volumes adjusted by weight: 150 mg/kg over 1h, 50 mg/kg over 4h, 100 mg/kg over 16h. Use 5% glucose. Concentrations adjusted for smaller fluid volumes (to avoid fluid overload in children <20 kg — use reduced volume protocols). TOXBASE guidance essential.

Dose adjustments

Renal

No dose adjustment required.

Hepatic

No dose adjustment (treatment for acute hepatic failure).

Paediatric weight-based calculator

3-bag regimen — volumes adjusted by weight: 150 mg/kg over 1h, 50 mg/kg over 4h, 100 mg/kg over 16h. Use 5% glucose. Concentrations adjusted for smaller fluid volumes (to avoid fluid overload in children <20 kg — use reduced volume protocols). TOXBASE guidance essential.

Clinical pearls

  • Start NAC even if >8 hours after ingestion if time uncertain or large overdose
  • Anaphylactoid reaction: STOP infusion, give chlorphenamine (10 mg IV) and antiemetic; restart at lower rate when settled
  • New 2-bag regimen (MHRA 2019): equal efficacy, fewer anaphylactoid reactions
  • If allergic reaction — paracetamol antidote must not be withheld — use methionine orally if truly impossible (TOXBASE)
  • Call NPIS (National Poisons Information Service — 0344 892 0111) for complex cases

Contraindications

  • Hypersensitivity to acetylcysteine (relative — anaphylactoid reactions are common but manageable)

Side effects

  • Anaphylactoid reaction (10–15%): flushing, urticaria, bronchospasm, nausea in first bag
  • Rash
  • Nausea and vomiting
  • Hyponatraemia (if inappropriate diluent used)

Interactions

  • No significant drug interactions

Monitoring

  • LFTs
  • INR
  • Creatinine/U&E
  • Glucose
  • Paracetamol level

Reference: BNFc; BNF; TOXBASE; MHRA Parvolex SPC 2019; NPIS guidelines. Verify against your local formulary and the latest BNF before prescribing.

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