WASHINGTON - The deaths of two patients prescribed a powerful painkiller as a headache treatment were among four fatalities linked to the recently approved drug, its manufacturer reported Thursday.

All four deaths apparently involved improper use of the drug, called Fentora, manufacturer Cephalon Inc. said. The Food and Drug Administration was monitoring the situation, a spokeswoman said.

The FDA approved the drug in September for use only by cancer patients already taking morphine or other prescription narcotics for their pain. Fentora contains fentanyl, which is similar to morphine, but far more potent.

Besides the two headache patients, the other deaths involved a suicide and a patient administered the drug outside the recommended dosing.

"None of the reports were in cancer patients, which leads us to believe they were inappropriate candidates for the product," Cephalon spokeswoman Candace Steele said.

Cephalon reminded doctors and pharmacists, in letters dated Monday but released Thursday by the FDA, of who should take the drug, and in what quantities and how often. The company also warned not to substitute Fentora on a one-for-one basis for another, older fentanyl drug called Actiq that it also makes, because the newer drug is stronger.

"The FDA takes this very seriously, and is working with the company to assure the safest possible use of this medication," agency spokeswoman Susan Cruzan said.

Steele said the deaths were not linked to allegations subject to investigation that Cephalon engaged in improper promotion of Fentora and other drugs for uses other than those outlined in their FDA-approved labels. While it's not illegal for doctors to prescribe drugs for so-called "off-label" uses, companies are prohibited from marketing medicines for those purposes.

Last month, Frazer, Pa.-based Cephalon said it was cooperating with the investigations by Congress, the U.S. attorney's office in Philadelphia and Connecticut attorney general.

Doctors have written about 78,000 prescriptions for Fentora tablets since October, Steele said.