Johnson & Johnson's Duragesic patch can provide up to three days' relief from severe chronic pain, such as that experienced by bone cancer patients. But fentanyl, its active ingredient, is highly dangerous.

An overdose of the morphine-like drug can put a patient into a coma and shut down breathing.

Fentanyl gained international notoriety in 2002, when Russian authorities trying to end a hostage crisis at a Moscow theater pumped in a gas reportedly containing the drug, intending to put the hostages and their Chechen captors to sleep.

Of the 750 hostages, about 120 died, nearly all from breathing the gas.

FDA officials said they are investigating whether the deaths among US patients could be the result of unintentional fentanyl overdose.

Such overdoses could come about if patients and doctors do not faithfully follow a series of precautions contained in the prescribing literature, or label, for the drug.

Other possible explanations include rare problems or defects with the patches themselves that would cause too much of the drug to be released into the body too quickly.

Doctors and patients should be aware of the signs of fentanyl overdose, which include trouble breathing or shallow breathing, tiredness, extreme sleepiness, inability to think, talk or walk normally and feeling faint, dizzy or confused, the FDA said. Patients experiencing these symptoms should get medical attention right away, the FDA said.

"The way people die is that it decreases the drive to breathe, so people will not breathe effectively" said Meyer. "It (can) make patients very sedated, or comatose."

FDA officials said the agency announced the investigation in keeping with its new commitment to give the public warning of possible problems with prescription drugs. The FDA has been strongly criticized for taking too long to respond to evidence of heart attack risks with Vioxx and other arthritis drugs, and to evidence of suicide risks for teenagers taking anti-depressants.

The fentanyl investigation is in its early stages, FDA officials said, and the agency has reached no decision on whether the patches should be recalled, or their use limited.

But the FDA did issue a new public health advisory Friday underscoring the need for patients to follow precautions for using the drug scrupulously.

"Some of the cases would seem to involve an overdose," said Dr. Robert Meyer, who heads the FDA office that regulates painkillers. "We understand that labeling is not always understood or adhered to, and this is a way to re-emphasize that these important warnings should be heeded."

A spokesman for New Jersey-based Johnson & Johnson said the company always has stressed that Duragesic should be used with great care.

"Patient safety is our first priority and it has been for the many years we have sold the drug," said Doug Stokke.

"We have consistently communicated to patients, caregivers and health care professionals information regarding the safe and appropriate use of Duragesic." Duragesic has been on the market since 1990, and the FDA said the 120 reported deaths span the entire period. Typically, the FDA's reporting system picks up only a fraction of serious drug reactions.

Mylan Laboratories, Inc. began marketing a generic version of the drug early this year.

Sales of brand-name Duragesic reached $1.2 billion in 2004, with more than 4 million prescriptions filled, according to the Web site drugtopics.com. It ranked 24th in dollar sales among the top 200 drugs in 2004, and 93rd in the number of prescriptions.

Because of its risks, Duragesic and its generic equivalent already carry a so-called "black box" warning -- the FDA's most emphatic. On Friday, the agency underscored those warnings.

Doctors should prescribe the lowest effective dose of the medication, said the agency. And fentanyl patches should not be used to treat short-term pain or pain after an operation.

Patients should not attempt to use any patches that are damaged or broken. And they should not drink alcoholic beverages or bask in the sun while taking the drug, since alcohol and a rise in body temperature can accentuate the narcotic effects.

Johnson & Johnson spokesman Stokke said the company recently added to the warnings, notifying doctors that Duragesic should not be used by patients who do not tolerate opioid drugs.

People using the patches would include certain cancer patients, those who have difficulty swallowing pills, and those who have problems taking drugs by injection.