WASHINGTON — The government issued a new safety warning Friday for a skin patch containing a potent painkiller that has been implicated in hundreds of deaths, saying the patch poses risks that doctors and patients often fail to understand.

The Food and Drug Administration said doctors are prescribing the widely used fentanyl patch to the wrong kinds of patients and the patches are being misused by patients who are unaware that something as routine as taking a hot shower while wearing the patch can trigger a potentially fatal overdose.

A medical journal study published this summer identified fentanyl, a narcotic as much as 100 times more powerful than morphine, as the suspect drug in more than 3,500 accidental deaths reported to the FDA from 1998 to 2005. Safety advocates said the agency’s latest warning, which echoes an alert issued in 2005, is too little, too late.

The patch was developed for cancer patients who suffer severe chronic pain and in some cases have trouble swallowing pills. But Dr. Bob Rappaport, head of the FDA division that oversees painkillers, said some doctors have been prescribing the patch to patients who don’t need such a powerful narcotic — and in at least one case even used it to treat headaches.

Patches can be more convenient than pills. The fentanyl patch is designed to deliver the drug at a steady rate for as long as three days, so patients don’t have to take medication several times a day. But there are also drawbacks. For example, a single patch contains a substantially larger total quantity of the medication than individual pills, and heat or exercise can increase the rate at which the drug is absorbed.

Rappaport said the FDA has not found such serious problems with other methods of delivering fentanyl, such as an injectable form of the drug.

“It’s a unique problem with patches because of … the way that the drug is delivered to the body and the way that it’s metabolized,” he said.

The FDA said doctors should not prescribe the patch for patients whose pain is expected to go away, such as those recovering from an operation. The patch should only be used for patients in chronic pain who are taking opiod-type drugs safely.