Cephalon Inc. said Tuesday that it has sued Watson Pharmaceuticals Inc. to prevent the generic-drug maker from selling a low-cost copy of the painkiller Fentora.

Watson, based in Corona, Calif., is seeking U.S. Food and Drug Administration approval to sell a generic of the drug, whose chemical name is fentanyl buccal. In its application, Watson challenged two patents owned by Cephalon that expire in 2019. Cephalon said the copy would violate its patents.

Cephalon produces several products at a Salt Lake City facility.

"We continue to believe that our patent position for this product is strong and intend to vigorously defend our intellectual property," Cephalon general counsel Jerry Pappert said Tuesday in a statement. The complaint was filed Monday in federal court in Wilmington, Del.

Fentora, a fast-acting tablet that dissolves in the mouth, is approved to relieve sudden episodes of cancer pain in adults already taking other opioids, or morphine-like drugs.

Overdose and use by patients without a tolerance for narcotics has led to at least five deaths since Fentora was approved in September 2006. Earlier this month, an FDA panel voted against wider use of the drug.

The drug generated $135 million in sales for Cephalon, based in Frazer, Pa., last year, or 7.6 percent of the company's revenue. Cephalon is seeking a court order to block approval of Watson's application until the patents expire.

In an April 24 statement, Watson said it believes it was the first to challenge the Cephalon patents. If so, the company could be rewarded with a six-month period in which it is the only generic-drug maker with FDA approval to sell the copy. Spokeswoman Patty Eisenhaur said the company had no comment on the lawsuit.

Under federal drug law, the lawsuit starts a time-period in which the FDA can't approve Watson's application for 30 months unless there's a court ruling in the generic-drug maker's favor before then.