Acetaminophen is an analgesic and antipyretic drug that has almost no antiinflammatory effects. Its mechanism of action is unknown, but it is thought to act centrally through inhibition of the enzymes cyclooxygenase (COX) type 3 (constitutive) and COX-2b (inducible) (see NSAIDs below). At therapeutic doses it has almost no side effects, provides effective analgesia for mild to moderate pain, and has an opioid-sparing effect for severe pain. 29 Acetaminophen has been available for oral and rectal use for many years, and recently an intravenous formulation has become available. Intravenous acetaminophen is likely to replace the acetaminophen prodrug, propacetamol, which is currently available for intravenous use in some countries. The oral bioavailability of acetaminophen is 100%, and the dose, whether oral, rectal, or intravenous, is 500 to 1000 mg 6 hourly. Overdose with as little as 10 g of acetaminophen can cause fatal hepatotoxicity. Acetaminophen should be withheld in patients with hepatocellular dysfunction.