Acetaminophen

The chemical compound acetaminophen or paracetamol is an analgesic (a pain killer).

In North America it is sold unter the trade name Tylenol® (McNeil-PPC, Inc) while it is known as Panadol in parts of Asia and Australasia. In some formulations acetaminophen is combined with the opiate codeine. It relieves headaches and other minor aches and pains and lowers fever. Its chemical formula is C₈H₉NO₂.

Acetaminophen belongs to the class of drugs known as non-steroidal anti-inflammatory drugs (NSAIDs); other members of this class include aspirin and ibuprofen (Advil®). Compared to aspirin, acetaminophen has the advantage of not causing stomach problems, but it doesn't fight inflammation nor does it thin the blood.

Common dosage is 500mg (up to 1000mg) every 4 hours for adults. Doses above 150mg/kg or 7.5g for an adult is likely to cause hepatotoxicity. Overdoses of acetaminophen can lead to liver failure and a painful death if untreated; because of the wide over-the-counter availability of the drug, it is often used in suicide attempts.

Treatment for acetaminophen overdose is the use of intravenous N-acetyl-cysteine[?].

Acetaminophen should not be taken after alcohol consumption, because the liver, when engaged in alcohol breakdown, cannot properly dispose of acetaminophen.

It was first synthesized in 1873 by Harmon Northrop Morse, it was not used medically until 1893. It did not gain popularity until after WW II when it was found to be a metabolite of phenacetin[?] (this fact was first discovered in 1899 and rediscovered in 1948), offering similar effects but markedly less toxicity.

The US patent on acetaminophen has expired (when?) and generic versions of the drug are widely available under the Drug Price Competition and Patent Term Restoration Act of 1984[?], although certain Tylenol preparations are protected until 2007.

Method of action

Acetaminophen, like the other drugs mentioned above, works by reducing the activity of the cyclo-oxygenase[?] 2 (COX) enzyme which produces prostaglandins. While the other drugs operate as competitive inhibitors[?] and directly block the active site of COX, acetaminophen blocks COX indirectly; this indirect blockage is effective in brain and endothelial cells but not in platelets and immune cells which have high levels of peroxides. This is the reason that aspirin fights inflammation and acts as a blood thinner while acetaminophen does not.

References:

• Olivier Boutaud et.al., Determinants of the cellular specificity of acetaminophen as an inhibitor of prostaglandin [H.sub.2] synthases, Proceedings of the National Academy of Sciences of the United States, May 14, 2002 v99 i10 p7130(6)