Acetaminophen may act predominantly by inhibiting prostaglandin synthesis in the central nervous system (CNS) and, to a lesser extent, through a peripheral action by blocking pain - impulse generation.
Acetaminophen probably produces antipyresis by acting centrally on the hypothalamic heat - regulating center to produce peripheral vasodilation resulting in increased blood flow through the skin, sweating, and heat loss.
The central action probably involves inhibition of prostaglandin synthesis in the hypothalamus.

Problems in humans have not been documented. Although controlled studies have not been done, it has been shown that acetaminophen crosses the placenta.
Risk - benefit must be considered.

Risk of hepatotoxicity with single toxic doses or prolonged use of high doses of acetaminophen may be increased in alcoholics or in patients regularly taking other hepatotoxic medications or hepatic enzyme inducers.
Chronic use of barbiturates (except butalbital) or primidone has been reported to decrease the therapeutic effects of acetaminophen.
Concurrent chronic, high-dose administration of acetaminophen with anticoagulants, cumarin or indandion-derivative may increase the anticoagulant effects.
Prolonged concurrent use of acetaminophen and a salicylate or NSAIDs is not recommended (may increase the risk of adverse renal effects).

Those indicating need for medical care:
Agranulocytosis (unexplained sore throat and fever) - anemia (unusual tiredness or weakness) - dermatitis, allergic (skin rash - hives, or itching) - hepatitis (yellow eyes or skin) - renal colic (pain in lower back) - renal failure (sudden decrease in amount of urine) - uremia - sterile pyuria ( cloudy urine) - thrombocytopenia (blood in urine or stools).

Signs and symptoms of overdose:
These signs may occur within the first hours after ingestion and persist for about 24 hours and include diarrhea, loss of appetite, nausea, vomiting, stomach cramp or pain, increased sweating; hepatotoxicity may occur 2 to 4 days after the overdose ingestion.

Usual adult dose:
325 - 650 mg every four hours, or 1 gram every six hours as needed, up to 4 grams daily (for short therm therapy).

Usual pediatric dose:
Infants up to 3 months of age - oral, 40 mg every four hours as needed.
Infants 4 to 12 months of age - oral, 80 mg every four hours as needed.
Children 1 to 2 years of age -oral, 120 mg every four hours as needed.
Children 2 to 4 years of age - oral, 160 mg every four hours as needed.
Children 4 to 6 years of age - oral, 240 mg every four hours as needed.
Children 6 to 9 years of age - oralc 320 mg every four hours as needed.
Children 9 to 11 years of age - oral, 320 to 400 mg every four hours as needed.
Children 11 to 12 years of age - oral, 320 to 480 mg every four hours as needed.