Acetylcysteine (NAC)
The antidote for paracetamol overdose, dosed by weight. Start based on the timed level and the treatment line.
On this page
21-hour (three-bag) regimen
| Bag | Dose | Over |
|---|---|---|
| 1 | 150 mg/kg | 1 hour |
| 2 | 50 mg/kg | 4 hours |
| 3 | 100 mg/kg | 16 hours |
Use a maximum weight of 110 kg for dose calculation. Many units now use the 12-hour SNAP regimen (100 mg/kg over 2 hours, then 200 mg/kg over 10 hours).
When to treat
- Level above the treatment line at 4 hours or later.
- Staggered overdose, unknown time, or presenting after 8 hours with a significant ingestion: treat without waiting for the level.
Cautions
- Anaphylactoid reactions (flushing, wheeze, low BP): pause, antihistamine, restart at a slower rate.
Frequently asked questions
Is acetylcysteine effective late?
It is most effective within 8 hours but still helps later and in liver failure.
Used in these guides
Sources
Doses on this page are typical doses from the sources above. Always confirm with a current drug reference, the product leaflet and your hospital protocol, especially in children, pregnancy and kidney or liver disease.
For FCPS Part 1
Revising for FCPS Part 1? Acetylcysteine is covered in our textbook chapter Analgesics and NSAIDs, with practice MCQs.