The warning comes after Cephalon earlier this month warned doctors about several patient deaths related to inappropriate prescribing of the drug, called Fentora.

Fentora is approved only for treating pain in cancer patients who experience breakthrough pain because they have developed tolerances to other opiate painkillers, such as morphine etc.

But many doctors prescribe the drug "off-label" for headaches and back pain. Once a drug has been approved by the FDA doctors are free to prescribe it as they see fit.

The FDA said that several Fentora-related deaths have occurred in patients who were prescribed the drug for off-label use.

It warned that Fentora should not be used for any other conditions. It should also only be given to patients who have developed opiate tolerance, as others will not be able to handle the high amounts of fentanyl contained in Fentora.

Finally, the administration warned people taking Fentora and their caregivers to carefully follow the drug's label directions. The agency also advised that patients and caregivers become familiar with the signs of fentanyl overdose.