Drug maker Cephalon this week issued a warning to doctors about deaths linked to its powerful painkiller Fentora, the FDA said.

Cephalon spokeswoman Candace Steele told Reuters news service that three deaths occurred during a fairly short period of time this summer and that they were most likely caused by respiratory failure.

The deaths were reported to have occurred in cases where Fentora was given to patients unable to tolerate opioid therapy, or in patients getting the drug inappropriately as a substitution for another drug, such as Fentora predecessor Actiq.

Fentora and Actiq, are approved by the FDA only for use in cancer patients, but are known to be prescribed by doctors as an “off-label” treatment for conditions such as headaches and back pain.

Cephalon’s marketing practices involving the two drugs are currently under investigation in three separate probes – investigations by the US attorney in Philadelphia and Connecticut attorney general and a congressional probe into off-label treatments.