Staggered paracetamol overdose — MRCEM SBA MCQ

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EasyToxicology and environmental emergenciesStaggered paracetamol overdoseMRCEM SBA

A 30-year-old woman presents to the emergency department at 18:00 after deliberately taking 24 paracetamol 500 mg tablets between 08:00 and 14:00. She weighs 60 kg, is alert and has no abdominal pain. Her plasma paracetamol concentration is 25 mg/L, ALT is 22 U/L and INR is 1.0. Which is the most appropriate treatment decision?

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Correct answer: A — Commence intravenous acetylcysteine now.

She reports taking 12 g of paracetamol—200 mg/kg—over six hours. This is a staggered overdose, not a single ingestion that can be assigned one time on the paracetamol treatment nomogram. UK medicines information indicates intravenous acetylcysteine for a staggered overdose irrespective of the measured paracetamol concentration; MHRA guidance advises giving it without delay rather than using the nomogram. Her normal ALT and INR do not justify waiting for evidence of liver injury. ([medicines.org.uk](https://www.medicines.org.uk/emc/product/3447/smpc)) **B** delays indicated treatment for a repeat concentration. **C** and **D** each assign an artificial single ingestion time to tablets taken across six hours; plotting from either end of that interval is inappropriate. Nomogram-based decisions would apply to an appropriately timed concentration after a discrete ingestion. **E** waits for biochemical injury rather than treating the reported overdose. She requires monitoring and reassessment during treatment, with repeat blood tests to guide its completion and a psychosocial assessment before discharge. ([medicines.org.uk](https://www.medicines.org.uk/emc/product/3447/smpc))

Reference: Treating paracetamol overdose with intravenous acetylcysteine: new guidance (Published 11 December 2014; guidance dated September 2012) — https://www.gov.uk/drug-safety-update/treating-paracetamol-overdose-with-intravenous-acetylcysteine-new-guidance Acetylcysteine 200 mg/ml Injection — Summary of Product Characteristics (Revised 28 August 2025) — https://www.medicines.org.uk/emc/product/3447/smpc