Paracetamol Overdose

Paracetamol overdose is the IMT clinical station's favourite toxicology scenario because the decisions are rule-based and the rules are published. The BNF's treatment summary, drawn from TOXBASE, tells you when to take the level, where the treatment line sits, and when acetylcysteine cannot wait.

Key takeaways

The BNF, summarising TOXBASE, defines an acute overdose as a potentially toxic dose taken in one hour or less; anything over more than an hour is staggered.
A plasma paracetamol level is only interpretable four hours or more after ingestion; the treatment line joins 100 mg/litre at 4 hours to 3.13 mg/litre at 24 hours.
Acetylcysteine is most effective within 8 hours of ingestion. If the level will be delayed 8 hours or more after an overdose of more than 150 mg/kg, start it without waiting.
The MHRA advises that every staggered overdose is treated with acetylcysteine without delay, and an unknown time of ingestion is managed as staggered.
For patients over 110 kg, calculate the mg/kg dose using 110 kg, not actual weight.
TOXBASE is the primary clinical toxicology database and the National Poisons Information Service is available day and night; say you would consult them when in doubt.

Why does paracetamol overdose matter so much on the medical take?

Because the patient looks well while the liver is being damaged. The BNF states that nausea and vomiting usually settle within 24 hours, that their return after two to three days with right subcostal pain usually indicates hepatic necrosis, and that liver damage is maximal three to four days after overdose.

The BNF also lists paracetamol among the delayed-action poisons for which patients should be admitted even if they appear well. Say that early, because it settles the admission question before the panel asks it.

How should you start the assessment?

Start with A to E and then take the history that decides everything: what was taken, how much, over what period, when the last tablet was, and whether anything else was taken with it. Weigh the patient, because every threshold is in mg/kg.

The BNF adds two weight rules worth saying aloud. For patients who weigh more than 110 kg, use 110 kg when calculating the dose ingested. For pregnant patients, use the pre-pregnancy weight.

Which patients need hospital assessment after an acute ingestion?

The BNF says serious toxicity is unlikely from a single ingestion of less than 75 mg/kg in under an hour, but that patients must be referred for medical assessment if any of four criteria applies.

The paracetamol was taken in the context of self-harm.
The patient is symptomatic.
75 mg/kg or more was taken in one hour or less.
The time of ingestion is uncertain but the dose is 75 mg/kg or more.

The BNF also says that although the benefit is uncertain, activated charcoal should be considered if the patient presents within one hour of ingesting more than 150 mg/kg.

When do you take the level, and how do you read it?

Take a single plasma paracetamol concentration at least four hours after ingestion, because the BNF is blunt that earlier samples cannot be interpreted. Plot it on the paracetamol treatment graph against the time since ingestion.

The reference treatment line joins 100 mg/litre (0.66 mmol/litre) at 4 hours and 3.13 mg/litre (0.02 mmol/litre) at 24 hours. A concentration on or above that line means acetylcysteine.

Send the bloods that change the decision with it: liver function, INR, renal function and a venous gas. The BNF says an ALT above the upper limit of normal, or an INR above 1.3 without another cause, is itself a reason to treat even below the line.

When must acetylcysteine start without waiting for the level?

When waiting would push treatment past the window in which it works best. The BNF states acetylcysteine is most effective within 8 hours of ingestion, after which effectiveness declines, though it still prevents or reduces liver damage up to and possibly beyond 24 hours.

The BNF's start criteria

Concentration on or above the treatment line.
Presentation within 8 hours of more than 150 mg/kg, if the level will be delayed 8 hours or more after the overdose.
Presentation 8 to 24 hours after more than 150 mg/kg or an unknown amount, or if symptomatic with jaundice or hepatic tenderness, even before the level is back.
Presentation after 24 hours with more than 150 mg/kg or an unknown amount, clear jaundice or hepatic tenderness, or a detectable paracetamol concentration.
Biochemical evidence of liver injury: ALT above normal or INR above 1.3, even with a level below the line.

Staggered overdose and unknown timing

A staggered overdose is a potentially toxic dose taken over more than one hour with possible self-harm intent. The BNF records the MHRA's advice that all such patients are treated with acetylcysteine without delay, and that an unknown time of ingestion is managed the same way.

Which acetylcysteine regimen, and when can you stop?

Two intravenous regimens exist. The BNF describes the standard 21-hour regimen, given as three consecutive infusions, and the modified 12-hour SNAP regimen, which is unlicensed and not endorsed by the MHRA but is recommended by the Royal College of Emergency Medicine and the National Poisons Information Service. Follow your trust's protocol and say so.

The stop rule is worth knowing precisely. The BNF says hepatotoxicity is unlikely if, at least 4 hours after the last ingestion, the patient has no symptoms of liver damage, the paracetamol concentration is below 10 mg/litre, ALT is normal and INR is 1.3 or less; acetylcysteine can then be discontinued.

In practice, name the safety net too: if there is any uncertainty about risk, the BNF says to seek advice from the National Poisons Information Service.

What is the trap in this scenario?

The trap is a reassuring level taken too early, or a single level applied to a staggered ingestion. A two-hour level cannot be plotted, and a patient who took tablets across the day has no single time-zero, so the nomogram does not apply to them.

The second trap is forgetting the person. A paracetamol overdose in the context of self-harm needs a risk assessment, a mental health referral before discharge, and a capacity assessment if the patient wants to leave before treatment is complete.

What if the patient refuses treatment or wants to leave?

Start from the presumption of capacity, as GMC Decision making and consent 2020 requires, and assess it for this specific decision: can they understand, retain, use and weigh the information about liver failure, and communicate a choice? A patient with capacity can refuse, and you document the discussion and keep the door open.

If capacity is in doubt, the GMC says to seek support from someone who knows the patient and, in complex cases, specialist input from psychiatry. Say who you would call, and that you would not let the patient walk out while that call was being made.

How this comes up at the IMT interview

At the IMT interview, paracetamol overdose is a station 2 clinical scenario of up to ten minutes, marked on investigations, diagnosis and management with communication marked throughout, followed by a one-minute handover. The clinical area is weighted 1.2 and each of two interviewers scores 1 to 5.

It also crosses into station 1 territory, because the same case can pivot to capacity, confidentiality or a relative asking questions. internalmedicineinterview's bank of over 361 questions rehearses both halves with AI-marked spoken practice.

FAQ

When is a paracetamol level interpretable?

Four hours or more after ingestion. The BNF, summarising TOXBASE, states that earlier samples cannot be interpreted. The level is plotted against time on the treatment graph, whose line runs from 100 mg/litre at 4 hours to 3.13 mg/litre at 24 hours.

What counts as a staggered paracetamol overdose?

A potentially toxic dose taken over more than one hour, with possible self-harm intent. The BNF records the MHRA's advice that all staggered overdoses are treated with acetylcysteine without delay, and that an unknown time of ingestion is managed as staggered.

How soon does acetylcysteine need to start?

It is most effective within 8 hours of ingestion. If the level will not be back within 8 hours of an overdose of more than 150 mg/kg, the BNF says to start acetylcysteine without waiting for it.

What is the SNAP regimen?

The modified 12-hour intravenous acetylcysteine regimen, also called the Scottish and Newcastle Acetylcysteine Protocol. The BNF says it is unlicensed and not endorsed by the MHRA, but is recommended by the Royal College of Emergency Medicine and the National Poisons Information Service. Follow local protocol.

When can acetylcysteine be stopped?

The BNF says clinically significant hepatotoxicity is unlikely if, at least 4 hours after the last ingestion, the patient has no symptoms suggesting liver damage, the paracetamol concentration is below 10 mg/litre, ALT is within the normal range and INR is 1.3 or less. Seek NPIS advice if uncertain.

Does a patient who has self-harmed automatically lack capacity?

No. GMC Decision making and consent 2020 says you must not assume a patient lacks capacity because of their behaviour, a medical condition including mental illness, or a choice you consider unwise. Assess capacity for the specific decision and seek specialist input if it is unclear.

What weight do you use for a heavy patient?

The BNF says to use 110 kg rather than actual body weight when calculating the mg/kg dose ingested by a patient who weighs more than 110 kg, so that a potentially toxic dose is not underestimated.