Quick answer: in the UK, a single acute paracetamol overdose is assessed with a paracetamol level taken 4 hours or more after ingestion and plotted on a single treatment line that runs from 100 mg/L at 4 hours to 15 mg/L at 15 hours. Anyone on or above the line gets acetylcysteine (NAC), with no risk-factor assessment. Start NAC without waiting for the level if the overdose is staggered, the timing is uncertain, or the patient presents 8 hours or more after a significant ingestion. Many UK emergency departments now use the 12-hour SNAP regimen.
Paracetamol overdose is one of the most common poisonings seen in UK emergency departments, and it is a regular MRCEM topic. Most questions test the thresholds, the timings and who needs treatment straight away. This guide covers the essentials. In practice, always check TOXBASE and your local protocol.
Why paracetamol is toxic
At therapeutic doses, most paracetamol is safely conjugated in the liver. In overdose, these pathways become saturated and more is converted to a toxic metabolite, NAPQI. Glutathione normally neutralises NAPQI, but once glutathione stores run out, NAPQI damages liver cells. Acetylcysteine works by replenishing glutathione, which is why it is most effective when given early.
The 2012 UK treatment line
In 2012, the Commission on Human Medicines (CHM) and MHRA simplified UK practice:
- A single treatment line joining 100 mg/L at 4 hours and 15 mg/L at 15 hours replaced the old “normal” and “high-risk” lines.
- Risk factor assessment was removed. Patients on or above the line receive NAC regardless of risk factors such as alcohol use or enzyme-inducing drugs.
- NAC is indicated regardless of the level when the overdose is staggered or there is doubt about the time of ingestion.
Levels taken less than 4 hours after ingestion cannot be interpreted, because absorption is still ongoing.
Management by time of presentation
| Presentation | Management |
|---|---|
| Within 1 hour of a significant ingestion | Consider activated charcoal if the airway is safe |
| Under 4 hours | Wait and take the paracetamol level at 4 hours, with baseline bloods. Start NAC if the level is on or above the line |
| 4–8 hours | Take the level straight away. NAC is most effective when started within 8 hours, so start it promptly if the level is on or above the line |
| 8–24 hours | Start NAC immediately if a significant amount was taken (for example over 150 mg/kg), without waiting for the level. Review once results are back |
| Over 24 hours | Treat if the patient is jaundiced, has liver tenderness or abnormal bloods. Otherwise, decide once results are back |
| Staggered overdose or unknown timing | Start NAC immediately, whatever the level |
Baseline bloods should include paracetamol level, U&Es, LFTs (ALT), INR, venous gas and glucose.
Acetylcysteine regimens
The SNAP regimen (12 hours)
Many UK hospitals now use the SNAP regimen, which gives the same total dose over a shorter time:
- Bag 1: 100 mg/kg over 2 hours.
- Bag 2: 200 mg/kg over 10 hours.
It causes fewer anaphylactoid reactions than the older regimen. It is used off-licence, under local protocols based on TOXBASE.
The licensed 3-bag regimen (21 hours)
- 150 mg/kg over 1 hour, then 50 mg/kg over 4 hours, then 100 mg/kg over 16 hours.
- Total dose: 300 mg/kg over 21 hours.
Weight cap: for patients over 110 kg, calculate the dose using 110 kg.
Reactions to acetylcysteine
Anaphylactoid reactions, such as flushing, itch, urticaria, wheeze or hypotension, are common, especially in the first bag. They are not true allergies.
- Pause the infusion and treat symptoms, for example with an antihistamine.
- Restart once symptoms settle, usually at a slower rate.
- A previous reaction is not a reason to withhold NAC.
When to stop
At the end of treatment, repeat the paracetamol level, ALT, INR and U&Es. Local protocols vary, but NAC is typically stopped when:
- the paracetamol level is below 10 mg/L,
- the ALT is normal or not significantly raised from admission,
- the INR is normal, and
- the patient has no symptoms of liver injury.
If these criteria are not met, NAC is continued at the second-bag rate and the bloods are rechecked.
Severe toxicity and liver referral
Discuss with a liver unit early if there are signs of acute liver failure. The King’s College criteria for paracetamol-induced liver failure are widely used:
- Arterial pH below 7.30 (or lactate above 3.0 mmol/L) after fluid resuscitation, or
- All three of: INR above 6.5 (PT over 100 seconds), creatinine above 300 µmol/L, and grade 3–4 encephalopathy.
Also look out for hypoglycaemia, rising INR and metabolic acidosis, which suggest severe toxicity.
Do not forget the psychosocial assessment
Most paracetamol overdoses are intentional. Every patient needs a risk assessment and mental health review once medically fit, in line with NICE guidance on self-harm.
Common exam traps
- Taking a level before 4 hours: it cannot be interpreted.
- Waiting for the level when you should not: staggered overdoses, uncertain timing and late presentations need NAC straight away.
- Old “high-risk” line: no longer used in the UK. There is one treatment line for everyone.
- Stopping NAC after a reaction: pause and restart instead.
- Correcting the INR with FFP: avoid this unless there is active bleeding, as the INR is an important marker of liver function.
Summary
- Take the level at 4 hours or later and plot it on the single treatment line (100 mg/L at 4 hours).
- Treat everyone on or above the line. Risk factors no longer matter.
- Start NAC immediately for staggered overdoses, uncertain timing and significant late presentations.
- SNAP regimen: 100 mg/kg over 2 hours, then 200 mg/kg over 10 hours. Cap the weight at 110 kg.
- Use the King’s College criteria to identify patients who need a liver unit.
Frequently asked questions
What is the paracetamol treatment line in the UK?
A single line joining 100 mg/L at 4 hours and 15 mg/L at 15 hours. Patients on or above it receive acetylcysteine.
When should acetylcysteine be started before the level is back?
For staggered overdoses, when the time of ingestion is uncertain, and when patients present 8 hours or more after a significant ingestion.
What is the SNAP regimen?
A 12-hour acetylcysteine regimen: 100 mg/kg over 2 hours, then 200 mg/kg over 10 hours. It causes fewer reactions than the older 21-hour regimen.
Is an acetylcysteine reaction an allergy?
No. It is an anaphylactoid reaction. Pause the infusion, treat the symptoms and restart.
Next steps
Our 20 free MRCEM SBA sample questions include a paracetamol overdose scenario. For more, explore our Toxicology resources and the Pharmacology and Poisoning course, or see the full question bank and mock tests in our membership plans.
Clinical content checked against the MHRA/CHM 2012 recommendations and current UK SNAP protocols on 1 October 2026. This article is for exam preparation. In clinical practice, always use TOXBASE and your local protocol.