In a letter from the drug manufacturer, Cephalon Inc., the company said the deaths were the result of improper dosing of its drug called Fentora.
FDA approved Fentora in September for use only by cancer patients already taking morphine or other prescription narcotics for their pain. The drug contains fentanyl, which is similar to morphine, but far more potent.
Cephalon also blamed the deaths on improper product substitution as well as using the drug in patients who can't tolerate opioids - a chemical substance that has a morphine-like action in the body and its main use is for pain relief.
There were four deaths including one suicide. "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 in a statement.
"None of the reports were in cancer patients, which leads us to believe they were inappropriate candidates for the product," Steele said in a statement.
Insisting that Fentora isn't a substitute for Actiq, another pain drug from Cephalon, or other fentanyl-containing products, the letter, dated Sept. 10 and sent to health-care professionals, also said the drug shouldn't be used to treat post-operative pain, acute pain, sports injuries or migraines.
"The FDA takes this very seriously, and is working with the company to assure the safest possible use of this medication," Steele said adding that the deaths were not linked to allegations subject to investigation.
Doctors have written about 78,000 prescriptions for Fentora tablets since October, Steele said.
The Frazer, Pennsylvania-based company said it is currently testing the use of Fentora for other types of pain. In August, Cephalon also said that it expected to seek FDA approval for expanding the approved uses for the drug by the end of this year.
