Shares of Cephalon (NASDAQ:CEPH) fell yesterday after the drugmaker announced that it had sent out a letter to health-care practitioners warning of the dangers of misusing one of its lead drugs, Fentora. The FDA has issued a warning letter.

The information, often called a "dear doctor" letter since it's addressed to physicians, noted several patient deaths from the use of the drug off-label or with inappropriate dosages.  

Fentora is an oral compound with the powerful opioid analgesic fentanyl as its active ingredient. It's used on-label to relieve cancer sufferers' intense pain. The drug was approved 12 months ago, and it was supposed to be Cephalon's answer to the sales shortfall once generic drugmakers such as Barr Pharmaceuticals (NYSE:BRL) entered the market with generic versions of Cephalon's pain lollipop Actiq.

The big question is what sort of impact the "dear doctor" letter and revised dosing information will have on the sales potential of Fentora. In the first six months of 2007, sales of Fentora have reached a solid $68 million. This represents less than 8% of Cephalon's $859 million in sales, but Fentora is one of Cephalon's keys for revenue growth in the coming years. The company has been attempting to expand the label for the drug into new pain-related indications.

Cephalon doesn't break out off-label sales for its products, so it's impossible to determine what portion of Fentora sales will be at risk from the "dear doctor" letter. Certainly the extra caution doctors will now exercise with the drug will limit its off-label usage, and it may make getting label expansions more difficult.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. Barr Pharmaceuticals is a recommendation of Motley Fool Stock Advisor newsletter. The Fool has an A+ disclosure policy