Fentora is approved to treat cancer patients whose pain is not sufficiently controlled by conventional painkillers like morphine or other powerful drugs.

The FDA said it is crucial for doctors to precisely follow prescribing instructions to avoid fatal overdoses of the drug, which should not be used for short-term management of migraines or headaches.

"FDA is monitoring this issue very closely," Steven Galson, the head of the agency's Center for Drug Evaluation and Research, said in a statement.

Doctors can prescribe drugs for uses outside their approved labeling, a practice known as "off label use." Cephalon is seeking FDA approval for use in a broader group of pain patients.

Last year, a probe by the Connecticut attorney general found that Cephalon promoted some drugs for uses for which they were not approved, a practice that is illegal.

Cephalon earlier this month detailed three deaths of patients on Fentora, which it said were due to inappropriate use of the drug.

The company on Wednesday said it is working with the FDA to finalize a new label to reflect new safety messages about the drug, which comes in pill form.

FASTER ACTING

In its letter to health professionals dated September 10, Cephalon said patient deaths were due to improper patient selection, dosing or product substitution. It also warned that Fentora should not be used as a replacement for a similar painkiller sold by Cephalon called Actiq.

Fentora delivers more medication to the blood than Actiq, and substituting the same dose for Actiq can be fatal, the FDA said.

Dr. Scott Fishman, past president of the American Academy of Pain Medicine and professor at the University of California in Davis, said a big issue is that doctors are not trained in pain management, especially with faster acting painkillers like Actiq and Fentora.

"There are a host of patients out there that need relief quickly. Doctors will feel compelled to use whatever works," Fishman said in an interview.

Fentora is made from fentanyl, which in raw form is 80 times stronger than morphine. Cephalon officials said once it is processed, the difference is much less.