Paracetamol (acetaminophen) poisoning is one of the most common causes of drug overdose worldwide and the leading cause of acute liver failure in many developed countries. Despite its excellent safety profile at therapeutic doses, paracetamol is highly hepatotoxic in overdose, and the widespread availability of the drug makes intentional and unintentional overdoses frequent occurrences. Understanding the toxic mechanism, applying appropriate risk assessment tools, and administering the specific antidote in a timely manner are essential for preventing serious outcomes.
Epidemiology of paracetamol poisoning varies by region but consistently accounts for a substantial proportion of poisoning-related hospitalizations and liver transplant referrals. In the United States, paracetamol is involved in approximately 50,000 emergency department visits, 25,000 hospitalizations, and 500 deaths annually. The majority of cases involve intentional self-poisoning, but unintentional overdoses — often from the concurrent use of multiple paracetamol-containing products — account for a significant minority and carry a higher risk of severe outcomes due to delayed presentation.
Toxic mechanism centers on the saturation of normal metabolic pathways. At therapeutic doses, the majority of paracetamol undergoes glucuronidation and sulfation to harmless metabolites, with only approximately 5 percent metabolized by CYP2E1 and CYP3A4 to the reactive intermediate N-acetyl-p-benzoquinone imine (NAPQI). NAPQI is normally rapidly detoxified by conjugation with hepatic glutathione. In overdose, the glucuronidation and sulfation pathways become saturated, a larger fraction of paracetamol is shunted toward NAPQI production, and hepatic glutathione stores become depleted. Once glutathione falls below approximately 30 percent of normal, NAPQI accumulates and covalently binds to cellular proteins, triggering mitochondrial dysfunction, oxidative stress, and centrilobular hepatic necrosis.
Clinical phases follow a characteristic temporal pattern. Phase I (30 minutes to 24 hours post-ingestion) is often asymptomatic or associated with nonspecific symptoms such as nausea, vomiting, diaphoresis, and pallor. Phase II (24 to 72 hours) marks the onset of hepatic injury, with right upper quadrant pain, elevation of transaminases, and prolongation of the prothrombin time. Phase III (72 to 96 hours) represents the peak of hepatic necrosis, with fulminant liver failure characterized by jaundice, coagulopathy, hepatic encephalopathy, cerebral edema, and multisystem organ failure. Phase IV (4 days to 2 weeks) is the recovery phase in patients who survive, with gradual resolution of hepatic dysfunction and regeneration of liver tissue.
Assessment begins with establishing the time and dose of ingestion, which should be considered unreliable and the worst-case scenario assumed. The Rumack-Matthew nomogram is used to stratify risk and guide treatment decisions following acute, single-timepoint ingestion of immediate-release paracetamol. The nomogram plots the serum paracetamol concentration against the time since ingestion. A concentration above the treatment line — which starts at 150 mcg per mL at 4 hours and declines exponentially — indicates a risk of hepatotoxicity and the need for N-acetylcysteine therapy. The nomogram cannot be used for sustained-release formulations, staggered ingestions over more than 24 hours, or cases where the time of ingestion is unknown.
N-Acetylcysteine (NAC) treatment is the specific antidote and dramatically reduces the risk of hepatotoxicity and death when administered early. The standard protocol involves a loading dose of 150 mg per kg infused over 60 minutes, followed by 50 mg per kg over 4 hours, then 100 mg per kg over 16 hours. An alternative 20-hour protocol is also used in some centers. NAC is most effective when initiated within 8 to 10 hours of ingestion, after which efficacy progressively declines. However, NAC should still be administered beyond this window in patients with established liver injury, as it provides benefits independent of early intervention. Adverse effects of NAC include anaphylactoid reactions, most commonly during the loading dose infusion, which can be managed by temporarily slowing the infusion rate and administering antihistamines.
Prognostic factors for severe outcomes include the dose ingested, the time elapsed before NAC initiation, the degree of transaminase elevation, the severity of coagulopathy, the development of metabolic acidosis, and the presence of hepatic encephalopathy. The King’s College criteria help identify patients who may require liver transplantation. Prevention strategies include limiting the maximum daily dose to 4 grams for adults (3 grams in higher-risk populations), education about the presence of paracetamol in combination products, unit-dose packaging, and the implementation of pack size restrictions in some countries.
