Acetaminophen belongs to a class of drugs called analgesics (pain relievers) and antipyretics (fever reducers). The exact mechanism of action of acetaminophen is not known. It may reduce the production of prostaglandins in the brain. Prostaglandins are chemicals that cause inflammation and swelling. Acetaminophen relieves pain by elevating the pain threshold, that is, by requiring a greater amount of pain to develop before a person feels it. It reduces fever through its action on the heat-regulating center of the brain. Specifically, it tells the center to lower the body's temperature when the temperature is elevated.

The FDA approved acetaminophen in 1951.

Acetaminophen is used for the relief of fever as well as aches and pains associated with many conditions. Acetaminophen relieves pain in mild arthritis but has no effect on the underlying inflammation, redness, and swelling of the joint. If the pain is not due to inflammation, acetaminophen is as effective as aspirin.

Acetaminophen is as effective as the non-steroidal anti-inflammatory drug ibuprofen (Motrin) in relieving the pain of osteoarthritis of the knee. Unless directed by a physician, acetaminophen should not be used for longer than 10 days.

When used appropriately, side effects with acetaminophen are not common.

The most common side effects are rash, nausea, and headache.

Other important side effects include:

• Hypersensitivity reactions
• Serious skin reactions
• Kidney damage
• Anemia
• Reduced number of platelets in the blood (thrombocytopenia)

Chronic alcohol use may also increase the risk of stomach bleeding. The most serious side effect is liver damage due to large doses, chronic use or concomitant use with alcohol or other drugs that also damage the liver.

Other serious side effects that have been reported include bleeding in the intestines and stomach, angioedema, Stevens-Johnson syndrome, and kidney damage. A reduction in the number of white blood cells has also been reported.

• The dose for adults is 325 to 650 mg every 4 hours or 500 mg every 8 hours when using immediate release formulations.
• The dose for extended release caplet is 1300 mg every 8 hours.
• The maximum daily dose is 4 grams.
• The oral dose for a child is based on the child's age and weight. If less than 12 years of age, the dosing is 10-15 mg/kg every 6-8 hours not to exceed 2.6 g/day (5 doses). If older than 12 years of age the dose is 40-60 mg/kg/day every 6 hours not to exceed 3.75 g/day (5 doses).

Acetaminophen is metabolized (eliminated by conversion to other chemicals) by the liver. Therefore drugs that increase the action of liver enzymes that metabolize acetaminophen, [for example, carbamazepine (Tegretol), isoniazid, rifampin (Rifamate, Rifadin, and Rimactane),] reduce the levels of acetaminophen and may decrease the effectiveness action of acetaminophen.

Doses of acetaminophen greater than the recommended doses are toxic to the liver and may result in severe liver damage. The potential for acetaminophen to harm the liver is increased when it is combined with alcohol or drugs that also harm the liver.

Cholestyramine (Questran) reduces the effect of acetaminophen by decreasing its absorption into the body from the intestine. Therefore, acetaminophen should be administered 3 to 4 hours after cholestyramine or one hour before cholestyramine .

Acetaminophen doses greater than 2275 mg per day may increase the blood thinning effect of warfarin (Coumadin, Jantoven) by an unknown mechanism. Therefore, prolonged administration or large doses of acetaminophen should be avoided during warfarin therapy.

Acetaminophen is excreted in breast milk in small quantities. However, acetaminophen use by the nursing mother appears to be safe.

Tylenol, Tylenol Arthritis Pain, Tylenol Ext, Little Fevers Children's Fever/Pain Reliever, Little Fevers Infant Fever/Pain Reliever, PediaCare Single Dose Acetaminophen Fever Reducer/Pain Reliever, Infants Feverall, Acephen, Neopap, and others.

What preparations of acetaminophen are available?

• Solution/suspension/syrup: 160 mg/5 ml
• Liquid: 500 mg/ml
• Chewable tablets: 80, 160 mg
• Tablet (disintegrating) 80, 160 mg
• Caplets: 325, 500, 650 mg
• Caplet (extended release): 650 mg
• Gelcaps: 500 mg
• Geltabs: 500 mg
• Suppository: 80, 120, 325, 650 mg

How should I keep acetaminophen stored?

Tablets and solutions should be stored at room temperature 15 C to 30 C (59 F to 86 F). Suppositories should be refrigerated between 2 C and 27 C (35 F and 80 F).

A prescription is not necessary for OTC acetaminophen, however, medicines that contain acetaminophen and another painkillers such as hydrocodone (Norco, Vicodin) or codeine (Tylenol #4) require a prescription.