Pharmacy Author: Omudhome Ogbru, PharmD
Medical and Pharmacy Editor: Jay W. Marks, MD

GENERIC NAME: acetaminophen

BRAND NAME: Tylenol and others

DRUG CLASS AND MECHANISM: 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. 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.

PRESCRIPTION: No.

GENERIC AVAILABLE: Yes.

PREPARATIONS: Liquid suspension, chewable tablets, coated caplets, gelcaps, geltabs, and suppositories. Common dosages are 325, 500 and 650 mg.

STORAGE: Store tablets and solutions at room temperature 15°-30°C (59°-86°F). Suppositories should be refrigerated below 27°C (80°F).

PRESCRIBED FOR: 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. It is as effective as the non-steroidal antiinflammatory drug ibuprofen (Motrin) in relieving the pain of osteoarthritis of the knee. Unless directed by physician, acetaminophen should not be used for longer than 10 days.

DOSING: The oral dose for adults is 325 to 650 mg every 4 to 6 hours. The maximum daily dose is 4 grams. The oral dose for a child is based on the child's age, and the range is 40-650 mg every 4 hours.

When administered as a suppository, the adult dose is 650 mg every 4 to 6 hours. For children, the dose is 80-325 mg every 4 to 6 hours depending on age.

DRUG INTERACTIONS: 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 (INH, Nydrazid, Laniazid), rifampin (Rifamate, Rifadin, Rimactane)] reduce the levels of acetaminophen and may decrease the 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) by an unknown mechanism. Therefore, prolonged administration or large doses of acetaminophen should be avoided during warfarin therapy.

PREGNANCY: Acetaminophen is used in all stages of pregnancy and is the drug of choice for short-term treatment of fever and minor pain during pregnancy.

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

SIDE EFFECTS: When used appropriately, side effects with acetaminophen are rare. 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. Chronic alcohol use may also increase the risk of stomach bleeding.

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