Reviewed by MedNext Clinical Team
Paracetamol (acetaminophen) is the most commonly used analgesic and antipyretic globally, available without prescription in most countries and found in hundreds of combination products. Despite its reputation for safety at therapeutic doses, paracetamol is responsible for more cases of acute liver failure in the United Kingdom and United States than any other drug [1]. Understanding its mechanism, correct dosing, and overdose risks is fundamental for clinicians and patients alike.
Mechanism of Action
The precise mechanism by which paracetamol produces analgesia and antipyresis remains incompletely understood. It inhibits prostaglandin synthesis centrally — acting preferentially on a COX-3 isoform in the brain and spinal cord — and modulates the endocannabinoid system via its active metabolite AM404. Unlike NSAIDs, it has minimal peripheral anti-inflammatory activity and does not inhibit platelet function or cause gastric mucosal injury [1].
Standard Dosing
Adults
The standard adult dose is 500–1,000 mg every 4–6 hours as required, with a maximum of 4,000 mg (4 g) in any 24-hour period. Doses should be spaced at least 4 hours apart. In patients with hepatic impairment, chronic alcohol use, or low body weight (under 50 kg), the maximum daily dose should be reduced to 2,000–3,000 mg to reduce hepatotoxicity risk.
Paediatric Dosing
Paediatric dosing is weight-based: 15 mg/kg per dose every 4–6 hours, up to a maximum of 60 mg/kg/day in children over 3 months. The maximum single dose should not exceed 1,000 mg. Neonatal dosing requires specialist guidance due to immature hepatic glucuronidation pathways. The MedNext Proprietary Clinical Dataset includes paediatric-specific monographs with age- and weight-adjusted dosing guidance.
Hepatotoxicity Risk
Paracetamol hepatotoxicity occurs via its reactive metabolite N-acetyl-p-benzoquinone imine (NAPQI), produced by CYP2E1 and CYP3A4. At therapeutic doses, NAPQI is rapidly detoxified by hepatic glutathione. When glutathione stores are depleted — through overdose, fasting, chronic alcohol use, or enzyme-inducing drugs — NAPQI accumulates and causes centrilobular hepatic necrosis [1].
Risk factors for paracetamol hepatotoxicity at lower doses include chronic alcohol misuse, malnutrition or fasting, enzyme-inducing drugs (rifampicin, carbamazepine, phenytoin, St John's Wort), and pre-existing liver disease. In these populations, the therapeutic window is narrowed and the maximum daily dose should be reduced.
Overdose Management
Paracetamol overdose is a medical emergency. The Rumack-Matthew nomogram is used to assess hepatotoxicity risk based on the plasma paracetamol concentration at a known time after ingestion. Treatment with N-acetylcysteine (NAC) — the precursor to glutathione — is highly effective when given early, ideally within 8–10 hours of ingestion, and should be initiated without delay when overdose is suspected [1].
Clinicians must be vigilant for staggered overdoses and delayed presentations, where the nomogram is less applicable. Any patient presenting with jaundice, coagulopathy, or encephalopathy following paracetamol ingestion requires urgent specialist hepatology input, as acute liver failure may necessitate transplant listing.
Combination Products: A Hidden Risk
Many over-the-counter cold and flu preparations, codeine combinations, and compound analgesics contain paracetamol. Patients taking multiple such products simultaneously may inadvertently exceed the maximum daily dose. Clinicians should always ascertain the full list of paracetamol-containing products a patient is taking before prescribing additional paracetamol.
References
- Prescott LF. Paracetamol: past, present, and future. Am J Ther. 2000;7(2):143-147.