Acetylcysteine

Why this drug is interesting

Paracetamol is metabolised largely by conjugation, with a small fraction oxidised by CYP2E1 to NAPQI, a highly reactive quinone imine. Glutathione detoxifies NAPQI. In overdose the conjugation pathways saturate, more NAPQI is formed, glutathione is exhausted, and NAPQI binds hepatocyte proteins and kills them. Acetylcysteine replenishes cysteine for glutathione synthesis, and also acts as a direct NAPQI scavenger and antioxidant.

What makes it interesting clinically is that the antidote works reliably and the delivery of it does not. Anaphylactoid reactions — vomiting, flushing, urticaria, bronchospasm — were common enough with the traditional regimen to interrupt treatment routinely, delay it, and occasionally cause it to be abandoned in a patient who needed it. The UK response was to change the infusion, not the drug.

The SNAP regimen

What the evidence showed

The SNAP randomised trial tested the modified regimen against the standard one. A subsequent observational study across three UK hospitals — 3,340 patients, 1,488 on the 21-hour regimen and 1,852 on SNAP — compared outcomes before and after implementation:³

• Antihistamine prescribing, used as a proxy for anaphylactoid reactions: 11.0% on the 21-hour regimen versus 2.0% on SNAP — an absolute risk reduction of 9.0% (95% CI 7.3–10.7)
• Peak ALT above 1,000 U/L: 4.3% versus 3.6% — an absolute difference of −0.7% (95% CI −2.1 to 0.6)
• Multivariable regression found the regimen was not a significant predictor of liver injury in any model

Dose calculation

• Use actual body weight, but apply a ceiling weight of 110 kg for obese patients¹
• 5% glucose is the preferred diluent; 0.9% sodium chloride may be used if glucose is unsuitable
• In children, reduce the fluid volume — the doses and regimen are the same as adults, but fluid overload is a real danger and the SmPC gives weight-banded volumes and rates

Anaphylactoid reactions

Features

• Flushing, urticaria, itch — the commonest, and often self-limiting
• Nausea and vomiting
• Bronchospasm — the feature that should be taken most seriously, particularly in asthmatics
• Hypotension, tachycardia
• Angioedema, rarely

Management, in outline

1. Stop or slow the infusion.
2. Treat the symptoms — an antihistamine for cutaneous features; nebulised bronchodilator for bronchospasm; consider adrenaline if the reaction is genuinely severe and the picture is one of anaphylaxis.
3. Restart at a slower rate once symptoms have settled. Most patients complete the course.
4. Record it as an anaphylactoid reaction to acetylcysteine, not as an allergy. An incorrect allergy label may deny this patient a life-saving antidote on a future occasion.

When to treat

The decision to start acetylcysteine — treatment lines, timing of levels, staggered and unknown-time ingestions, high-risk features — is set by TOXBASE, and is beyond what a drug monograph should attempt to summarise. UK practice changed substantially after the 2012 MHRA review, which lowered the treatment threshold to a single line and removed the high-risk-group distinction.

Two further points worth carrying: treatment should not be delayed waiting for a level where the history suggests a substantial ingestion and the timing is late, and acetylcysteine retains benefit in established hepatic failure, well beyond the window in which it prevents injury.

Pharmacology

Mechanism

• Glutathione repletion — provides cysteine, the rate-limiting substrate for glutathione synthesis
• Direct NAPQI conjugation — acts as a glutathione substitute at high concentrations
• Increased sulfation of paracetamol, diverting it from oxidative metabolism
• Non-specific antioxidant and microcirculatory effects, which are the proposed basis for its benefit in established liver failure

Acetylcysteine is also used, orally or nebulised, as a mucolytic, and intravenously has been studied for contrast-induced nephropathy prophylaxis — an indication that has not held up in adequately powered trials.

Critical appraisal

1. SNAP is a rare example of an intervention improved by making it more convenient. Twelve hours instead of twenty-one reduces reactions, shortens admission and frees a bed, with no signal of reduced efficacy. Improvements of that shape are unusual and worth noticing.
2. The licence lags the practice, which is a documentation problem rather than a clinical one. A regimen backed by a randomised trial, a large implementation study, an age-stratified secondary analysis and the national poisons service, but absent from both UK product labels, means the SmPC will not answer the question a clinician actually has. That is worth knowing when checking a prescription against a label — it is not a reason for hesitation about the regimen itself.
3. The efficacy evidence for SNAP is observational. The reaction reduction is large enough to be beyond reasonable doubt. The hepatic non-inferiority rests on a before-and-after comparison whose confidence interval leaves room for a small difference — worth stating plainly rather than presenting as equivalence.
4. Anaphylactoid reactions are still routinely mislabelled as allergy. Every such label is a future patient who may be denied an antidote, and correcting the record is part of treating the reaction.
5. The nomogram is applied well outside its validated conditions. Staggered and unknown-time ingestions are common presentations, and reaching for the graph is a reflex. Recognising when it does not apply is more clinically useful than knowing where the line sits.

References

1. 1
   Acetylcysteine 200mg/ml Injection — Summary of Product Characteristics, Martindale Pharma. electronic Medicines Compendium. Sections 4.2, 4.4, 4.8, including the adult and paediatric prescription tables and the 110 kg ceiling weight. Verified 24 Aug 2026. Both UK intravenous products describe the 21-hour regimen only — see also Parvolex, Phoenix Labs, emc, text revised 2017. Neither label mentions a 12-hour regimen.
2. 2
   Bateman DN, Dear JW, Thanacoody HKR, et al. Reduction of adverse effects from intravenous acetylcysteine treatment for paracetamol poisoning: a randomised controlled trial (SNAP). Lancet 2014;383(9918):697–704. PubMed
3. 3
   Pettie JM, Caparrotta TM, Hunter RW, et al. Safety and efficacy of the SNAP 12-hour acetylcysteine regimen for the treatment of paracetamol overdose. eClinicalMedicine 2019;11:11–17. PMC Source of the 11.0% versus 2.0% antihistamine figures and the ALT comparison quoted above.
4. 4
   Humphries C, Pettie J, Agboola B, et al. Scottish and Newcastle Antiemetic Protocol (SNAP) 12-hour acetylcysteine regimen for paracetamol overdose reduces anaphylactoid reactions without compromising hepatic protection in all age groups: a secondary analysis. Emerg Med J 2025;43(1):e214533. PMC Source of the age-band figures above; population aged 13 and over.
5. 5
   TOXBASE — paracetamol; acetylcysteine. National Poisons Information Service. toxbase.org (NHS login required. NPIS: 0344 892 0111. The authoritative UK source for treatment thresholds, staggered ingestion and the regimen to use.)
6. 6
   Medicines and Healthcare products Regulatory Agency. Treatment of paracetamol overdose: simplified new guidance. Drug Safety Update, 2012. gov.uk