Paracetamol Overdose: Liver Risk & Treatment

Why Paracetamol Is So Dangerous in Overdose

In normal doses, paracetamol is metabolised safely by the liver. In overdose, a toxic metabolite (NAPQI) overwhelms the liver's glutathione stores and starts destroying liver cells. The dangerous part: people often feel fine for the first 24 hours while silent liver damage occurs.

Timeline of Paracetamol Overdose

Blood Tests Used in A&E

References

Sources cited on this page. PubMed links open the original abstract.

1. Larson AM, Polson J, Fontana RJ, et al. Acetaminophen-induced acute liver failure. Hepatology. 2005;42(6):1364–1372. PMID 16317692 · doi:10.1002/hep.20948

The mechanism of paracetamol toxicity – why therapeutic doses are safe but overdose is dangerous

Paracetamol (acetaminophen) is metabolised in the liver primarily by conjugation with glucuronide and sulphate – safe, water-soluble metabolites excreted in urine. A small proportion (approximately 5%) is metabolised via the cytochrome P450 enzyme CYP2E1 to NAPQI (N-acetyl-p-benzoquinoneimine) – a highly reactive, toxic metabolite. At therapeutic doses, NAPQI is immediately neutralised by hepatic glutathione.

In overdose, the conjugation pathways become saturated. A disproportionately large amount of paracetamol is diverted through CYP2E1, generating NAPQI faster than glutathione can neutralise it. Accumulated NAPQI covalently binds to liver cell proteins, triggering mitochondrial dysfunction, oxidative stress, and hepatocyte death – acute liver necrosis.

This process has a crucial "silent period": for the first 24 hours after overdose, the patient may feel well or have only mild nausea. Liver failure – if it occurs – begins to become apparent at 24–72 hours. This is why paracetamol overdose is clinically treacherous: apparent early wellbeing can falsely reassure both patient and clinician.

The paracetamol treatment nomogram and NAC

N-acetylcysteine (NAC) is the antidote for paracetamol overdose. It replenishes hepatic glutathione stores, directly neutralising NAPQI before it can damage hepatocytes. NAC is highly effective if given within 8–10 hours of overdose; efficacy decreases but benefit persists even up to 24 hours in severe overdose.

Treatment decisions use a paracetamol concentration versus time nomogram (the Rumack-Matthew line in North America; the UK uses a single line at 100 mg/L at 4 hours, declining linearly). A 4-hour post-ingestion plasma paracetamol level above 100 mg/L (UK threshold) or above 150 mg/L (USA threshold) indicates NAC treatment. Below these thresholds, NAC is not required. Levels cannot be interpreted before 4 hours (peak absorption incomplete); levels after 24 hours may be undetectable even in severe overdose – at this stage, liver function tests and INR guide management regardless of paracetamol level.

Patients at increased risk (those on enzyme-inducing drugs such as phenytoin, rifampicin, or carbamazepine; those with chronic alcoholism; or those severely malnourished) have reduced glutathione stores and greater CYP2E1 induction – they can develop liver damage at lower paracetamol concentrations. Previously, lower thresholds were used in these groups, but the UK switched to a single universal threshold in 2012 because the "high-risk" line did not improve outcomes and caused treatment of patients who did not benefit.

Monitoring after paracetamol overdose

Blood tests are used to track whether liver injury is progressing and whether emergency liver transplant assessment is needed:

• ALT / AST: Begin rising at 24–36 hours; peak at 72–96 hours. An ALT above 1000 IU/L indicates significant hepatotoxicity. In severe cases, ALT can exceed 10,000 IU/L.
• INR: The most sensitive prognostic marker. INR above 2 at 24 hours or above 3.5 at 48 hours are criteria triggering liver transplant unit referral. An INR above 6.5 at any time is a King's College criteria for transplant listing.
• Creatinine: Hepatorenal syndrome – kidney failure secondary to liver failure – worsens prognosis. Creatinine above 300 µmol/L is another King's College criterion.
• Blood glucose: Severe liver failure impairs gluconeogenesis – hypoglycaemia is a late and ominous sign of fulminant hepatic failure.