By Susan Heavey

WASHINGTON (Reuters) - Allowing Cephalon Inc (CEPH.O: Quote, Profile, Research) to market its Fentora pain drug more widely for patients without cancer could have "devastating effects," a U.S. Food and Drug Administration official said in documents released on Friday.

The company is seeking the FDA's approval to promote Fentora, or fentanyl, for sudden pain in non-cancer patients who have developed a tolerance to opioids. The drug is already approved in the United States for pain flare-ups in cancer patients.

The FDA's pending review of the new use has already been weighing on Cephalon shares, which fell more than 3 percent following the FDA's comments.

"We are concerned that the sponsor's request to expand the current indication for Fentora ... may greatly increase the prescribing of this product which may increase the availability of the product for diversion, abuse and misuse," wrote Bob Rappaport, head of the FDA's Division of Anesthesia, Analgesia and Rheumatology Products.

Rappaport added that could also "increase the incidence of accidental exposures which, due to the potency of the product, could potentially have devastating effects."

Narcotic drugs, including opioids such as Fentora, are highly addictive and sought out for their calming effects. Even patients with pain can become dependent and abuse them.

Rappaport's comments were released ahead of an FDA advisory meeting on Tuesday to discuss the issue with a panel of outside experts. The agency is set to ask the panel whether wider use should be approved, according to the documents, although the FDA will make the final decision later.

Last September, the FDA issued a pubic warning over Fentora following reports that its use has led to deaths as well as breathing problems and other serious complications.