Health and Human Services Secretary Tommy Thompson said the action was part of an effort to boost production of drugs that could be used in the event of another terrorist attack, especially one involving radioactive materials.

The FDA said Prussian blue, which binds with radioactive particles and expels them from the body, would be the first therapy available to decrease radiation exposure. The main side effects were constipation and stomach upset.

It said the drug could be used to treat patients with known or suspected internal contamination with radioactive thallium, non-radioactive thallium, or radioactive cesium.

FDA said cesium-137, found in the fallout from the detonation of nuclear weapons and in the waste from nuclear power plants, was of particular concern because it could potentially be use to build a dirty bomb. Exposure to cesium can cause serious illness and possibly cancer.

TREATMENT IN BRAZIL

An agency spokeswoman said Prussian blue is produced in the United States, but not under pharmaceutical standards. She said it has been used experimentally since the 1960s as an orally ingested drug to increase fecal excretion of cesium and thallium without it being absorbed through the intestines.

There are no other FDA-approved treatments for contamination with thallium or radioactive cesium, FDA said.

Prussian blue was used to treat 250 people in Brazil in 1987 after they were contaminated with cesium-137 abandoned after use in a cancer clinic, helping expel the radioactive materials more quickly.

U.S. authorities are holding Jose Padilla, a U.S. citizen who was captured in May and is accused of plotting with al Qaeda to detonate such a device.

A so-called "dirty bomb" involves exploding a conventional bomb wrapped in radioactive material that can kill victims in the immediate area and spread highly toxic material to humans, causing mass death and injury.