Doctors have discovered that a safe readily available drug can prevent liver damage and death in people who have taken an overdose of the common household painkiller acetaminophen.

Acetaninophen is widely used as a substitute for aspirin. It is marketed under such trade names as Tylenol, Datril, Tempra and Liquiprin, and is included in a number of combination products, such as Excedrin.

The antidote is acetyleysteins or Mucomyst, which is commomly used to aid coughing in patients with lung problems. Given up to 16 hours after an acetaminophen overdose, it helps rid the body of the drug, which can otherwise cause jaundice, bleeding and fatal liver failure, according to a report in this week's issue of "The Medical Letter," a newsletter for doctors.

Acetaminophen is a pain and fever treatment that has gained popularity as an alternative to aspirin. As a result, acetaminophen overdoses, intentinal and accidental, have become common in emergency rooms.

No national figures are available, but one multi-hospital study found more than 2,500 overdoses in the last three years. Fourteen percent of the patients had blood levels of the drug high enouh to cause liver damage, and three patients died, said Dr. B. H. Rmack, professor of pediatrics at the University of Colorado School of Medicine.

Rumack and other pediatricians decided to look for an antidote because acetaminophen overdoeses are most common in adolescents. Children under 5 are also a high-risk group, but for some reason they handle the drug better than older people, and seldom have liver damage, he said.

The doctors chose acetylcysteine because it is chemically similar to gultathione, the substance naturally used by the body to detoxify the drug. Research suggested that liver damage begins when the body's glutathione store is used up, so Rumack and others reasoned that adding to the store might work.

They found that if they started giving acetylcysteine by mouth any time up to 16 hours after the overdose, they could completely prevent liver damage. The drug still provided some benefit up to 24 hours after the overdose.

Rumack said an average-sized adult has to take about 16 grams, 50 regular tablets, of acetaminophen to achieve a dangerously high level in the blood-stream. But he said that because the antidote is safe, and because doctors cannot usually be sure what dose a patient took, the antidote should be given without waiting for blood test results.