Acetaminophen, also known as paracetamol, is a nonsteroidal anti-inflammatory drug with potent antipyretic and analgesic actions but with very weak anti-inflammatory activity. When administered to humans, it reduces levels of prostaglandin metabolites in urine but does not reduce synthesis of prostaglandins by blood platelets or by the stomach mucosa. Because acetaminophen is a weak inhibitor in vitro of both cyclooxygenase (COX)-1 and COX-2, the possibility exists that it inhibits a so far unidentified form of COX, perhaps COX-3. In animal studies, COX enzymes in homogenates of different tissues vary in sensitivity to the inhibitory action of acetaminophen. This may be evidence that there are >2 isoforms of the enzyme. Recently, a variant of COX-2 induced with high concentrations of nonsteroidal anti-inflammatory drugs was shown to be highly sensitive to inhibition by acetaminophen. Therefore COX-3 may be a product of the same gene that encodes COX-2, but have different molecular characteristics.
Acetaminophen, known as paracetamol in the United Kingdom, was introduced to medicine in 1893 [1]. It had only limited use, however, until 1949, when it was identified as the active metabolite of 2 older antipyretic drugs, acetanilide and phenacetin [2, 3]. Its popularity as an analgesic and antipyretic gradually increased, but it was not marketed in the United States until 1955, by McNeil Laboratories, and it is now the best-selling analgesic under the trade name of Tylenol. Recent clinical studies [4, 5] did not show any advantage of analgesic or anti-inflammatory doses of ibuprofen over acetaminophen as symptomatic treatment for patients with osteoarthritis [6]. In addition, the side effects of long-term administration of acetaminophen are less severe, without the gastrotoxicity of most nonsteroidal anti-inflammatory drugs (NSAIDs). Acetaminophen-induced liver damage is normally seen only with daily doses greater than 10 g, whereas the recommended therapeutic dose for adults is 4 g [7].
In spite of its wide use, the mechanism of action of acetaminophen has not been fully elucidated. It is only a weak inhibitor of prostaglandin (PG) synthesis in vitro and appears to have very little anti-inflammatory activity, although some reduction of tissue swelling after dental surgery has been reported [8, 9]. Because of its potent analgesic and antipyretic actions, it is generally regarded as an NSAID. However, it lacks the other typical actions of NSAIDs, such as antiplatelet activity and gastrotoxicity.