The SNAP regimen for paracetamol overdose

Paracetamol overdose is a common FRCEM SBA topic: common, protocol-driven, and full of the small details that make for clean single-best-answer questions.

RCEM now recommends moving from the old 21-hour, three-bag acetylcysteine regimen to the shorter SNAP regimen, so it is essential candidates are familiar with this ahead of the exam.

Written by the Bolus editorial team. Last reviewed 14 July 2026. This article is for exam revision and does not replace TOXBASE or local clinical guidance.

What is the SNAP regimen?

SNAP is the Scottish and Newcastle Acetylcysteine Protocol. It is a 12-hour, two-infusion acetylcysteine regimen used for paracetamol overdose:

This gives the same total acetylcysteine dose as the traditional 21-hour regimen: 300 mg/kg. The advantage is not that it is more effective, but that it is simpler and better tolerated, with fewer anaphylactoid reactions and a shorter treatment time.

The key exam point: 12 hours is the minimum

SNAP is a 12-hour initial regimen, not a guaranteed 12-hour course. Bloods are repeated towards the end of the second infusion, and acetylcysteine is only stopped if the biochemical picture is reassuring.

Published SNAP stopping criteria include a low or undetectable paracetamol concentration, acceptable INR, and an ALT that is normal or not rising significantly. If the criteria are not met, treatment continues with an additional acetylcysteine infusion, usually at the same dose and rate as the second infusion.

What hasn't changed

The fundamentals are the same. For a single acute ingestion with a reliable time of overdose, the paracetamol level is plotted on the treatment nomogram, and that level must be taken at 4 hours or later. A level drawn before 4 hours can't be plotted reliably.

The nomogram doesn't apply in the usual way to staggered overdoses, uncertain ingestion times, late presentations, therapeutic excess, or patients who already have symptoms or deranged bloods. These are exactly the scenarios exam writers use to make a question harder.

A note on massive overdoses

One area worth being aware of is the very large, or "massive", overdose. Where the paracetamol concentration sits well above the nomogram line, current UK guidance may recommend an increased acetylcysteine dose rather than the standard regimen.

Common FRCEM traps

• Plotting a level taken too early. A pre-4-hour level is a classic distractor. It can look reassuring but can't exclude toxicity.
• Waiting for a level when treatment should already be running. With a late or staggered presentation, an uncertain time, or signs of liver injury, acetylcysteine is usually started while assessment continues.
• Treating SNAP as a guaranteed 12-hour course. It's a 12-hour initial regimen, not a fixed-length one (see the stopping criteria above).
• Revising from international sources. UK thresholds and practice differ from some overseas guidance. For the FRCEM SBA, UK guidance is what counts.
• Ignoring assay interference. Some analysers underestimate paracetamol concentrations in samples taken during an acetylcysteine infusion. An unexpectedly low level mid-treatment may be testing exactly this.

Quick exam summary

For the wider toxicology you need across the exam, see our guide to high-yield FRCEM SBA guidelines, which sits within our complete FRCEM SBA study guide.

Frequently asked questions

Is SNAP the same dose as the old acetylcysteine regimen?

Yes, 300 mg/kg total. SNAP simply delivers it over 12 hours in two infusions instead of 21 hours in three.

Can acetylcysteine be stopped after 12 hours?

Only if the stopping criteria are met. If the bloods aren't reassuring, it continues.

What's the most common exam trap?

Using a paracetamol level taken before 4 hours, or applying the nomogram to a staggered or uncertain overdose.

This article is for exam preparation only and is not a clinical protocol. Real patients should be managed using TOXBASE and current local and national guidance.