Paracetamol Poisoning: NAPQI and the N-Acetylcysteine Rescue

The therapeutic dose: mostly harmless

At normal doses, the liver disposes of paracetamol quietly and completely. Swallow a normal dose and the liver deals with it by phase II conjugation — the same machinery covered in the Principles of Pharmacology chapter. The great majority of the drug is coupled to glucuronide or sulfate, turned into a harmless water-soluble compound, and passed out in the urine. Only a small fraction takes a different, more dangerous road: it is oxidised by the cytochrome enzyme CYP2E1 into a highly reactive metabolite called NAPQI (N-acetyl-p-benzoquinone imine). NAPQI is a toxin. But at therapeutic doses it is produced in such tiny amounts that the liver's stock of glutathione — a small protective thiol molecule — instantly grabs it, neutralises it, and excretes it safely. The poison is made every time you take the drug; it simply never accumulates.

Overdose: the pathways saturate and NAPQI wins

In overdose the arithmetic breaks. The glucuronidation and sulfation pathways are high-capacity but finite; flood them with paracetamol and they saturate — this is the conjugation saturation the Principles of Pharmacology chapter describes for phase II metabolism. With the safe roads jammed, a far larger share of the drug is diverted down the CYP2E1 route, and NAPQI is produced in bulk. At first glutathione keeps mopping it up, but the stores are limited: once glutathione falls to roughly 30% of normal, the mop runs dry. Now free, unquenched NAPQI does what a reactive electrophile does — it binds covalently to the sulfhydryl groups of hepatocyte proteins, wrecking mitochondrial function and triggering cell death. The damage is concentrated in the centrilobular (zone 3) region of the liver, where CYP2E1 is richest, producing the characteristic centrilobular hepatic necrosis. In severe cases the kidneys, which also carry CYP2E1, are injured too.

The four deceptive phases

The natural history of the poisoning is a trap built out of feeling well. Untreated paracetamol poisoning moves through four phases, and the first is the one that kills. Phase 1 (0–24 hours): the patient is well or has only mild nausea and vomiting — this is the treacherous latent period, when the level may be lethal yet the patient looks and feels normal. Phase 2 (24–72 hours): the patient may seem to improve, but the liver is now injured — right upper quadrant pain and tenderness appear and the transaminases (ALT, AST) climb, often to enormous heights. Phase 3 (72–96 hours): the peak of hepatotoxicity — fulminant hepatic failure with jaundice, coagulopathy, encephalopathy, hypoglycaemia and lactic acidosis; this is when patients die of liver failure or need a transplant. Phase 4 (4 days–2 weeks): survivors recover, and the liver — if it survives — regenerates completely, usually without chronic scarring. The lesson echoes the "lethal latent period" theme of the Approach to the Poisoned Patient chapter: in toxicology, a well-looking patient is not a reassuring patient.

Risk assessment: the timed level and the nomogram

Because the patient's appearance is worthless as a guide, risk is decided by numbers. The cornerstone is a paracetamol concentration measured at a known time after a single acute ingestion, taken no earlier than 4 hours (before that, absorption is incomplete and the level is uninterpretable). The value is plotted on the Rumack–Matthew nomogram — a graph of paracetamol concentration against hours since ingestion, with a treatment line running down it. A level on or above the line means significant risk of hepatotoxicity: treat. The nomogram is a beautiful tool but it has strict limits: it is valid only for a single, well-timed acute overdose taken within the previous 24 hours. It is useless for staggered overdoses (repeated supratherapeutic doses spread over hours or days) and for presentations where the timing is unknown or beyond 24 hours — in all of these, you cannot rely on the graph and should treat based on dose history, symptoms and blood results instead. A special trap is modified-release paracetamol, whose prolonged absorption can push the level back above the line after an initially reassuring reading, so a single early level can falsely reassure.

The antidote: N-acetylcysteine

N-acetylcysteine is the model antidote — it works precisely by refilling the tank the poison emptied. N-acetylcysteine (NAC) attacks the problem at its root. It is a precursor for glutathione synthesis, so it replenishes the depleted glutathione stores and lets the liver resume detoxifying NAPQI; it also provides sulfhydryl groups that can bind and neutralise NAPQI directly; and at high levels it enhances the safe sulfate conjugation pathway. The single most important fact about NAC is that it is time-critical: given within roughly 8 hours of ingestion it is almost completely hepatoprotective — patients treated this early very rarely develop serious liver injury. Its efficacy falls the longer it is delayed, because by then NAPQI has already bound its targets. Two management principles follow. First, if the timing is uncertain, or the level will not return before that 8-hour window closes, give NAC empirically and stop later if the risk assessment turns out reassuring — you never withhold a near-perfect antidote to wait for a number. Second, NAC still has value late: even in established hepatic failure it improves outcomes, so "too late for the nomogram" is not "too late for NAC." This mechanism-first thinking belongs to the Enhanced-elimination & Antidotes chapter, where NAC stands as the archetype of an antidote that restores a depleted physiological defence rather than blocking a receptor.

NAC has one notorious quirk students must expect: anaphylactoid reactions. During the infusion — especially early, when the concentration is highest — patients commonly develop flushing, urticaria, itch, and sometimes wheeze. Crucially, these are non-allergic (anaphylactoid, histamine-driven) reactions, not true IgE allergy, and they are dose-rate dependent. The correct response is almost never to abandon the antidote: the infusion is paused or slowed and an antihistamine given, then NAC is restarted. Stopping a life-saving antidote because of a manageable flush is a classic error. Alongside NAC, general poisoning care applies: activated charcoal can be considered if the patient presents within an hour of a significant ingestion, and supportive care manages the hepatic failure.

When the liver can't be saved: transplant referral

A minority progress to fulminant hepatic failure despite treatment, and here the decision is whether the liver can recover on its own or whether the patient needs an emergency transplant. The King's College Criteria are the classic tool for flagging patients whose prognosis without transplant is grim: an arterial pH below 7.3 after adequate fluid resuscitation, or the combination of a high prothrombin time / INR, a raised creatinine, and grade III–IV hepatic encephalopathy. Meeting these criteria triggers urgent referral to a liver transplant unit. Rising lactate and a persistently climbing INR are the ominous trends. The point for a pharmacology learner is not to memorise the thresholds but to understand what they encode: they identify the patient in whom the poison has outrun both the native liver and the antidote.