Shares in the US firm Cephalon have fallen 5.2% after US regulators issued a warning about the firm’s analgesic Fentora, which is approved for the treatment of breakthrough pain in cancer patients.

The US Food Drug Administration has issued a product safety alert to healthcare professionals after it received information of serious adverse events, including deaths, that have occurred in patients treated with Fentora (fentanyl).

Cephalon, which collaborated with the FDA on the letter, said that the events were a result of improper selection, namely use in opioid non-tolerant patients, incorrect dosing, and/or improper product substitution of Fentora for other fentanyl-based medications. The Pennsylvania-based company noted that Fentora should not be substituted for its own Actiq or other fentanyl-containing products, or be prescribed for people who have acute or postoperative pain, headaches or migraine or sports injuries.

Cephalon added that only one tablet per episode should be taken once a dose has been established and patients must wait at least four hours before taking Fentora again. It also noted that it is “proactively working with the FDA to emphasize the appropriate patient selection, dosing and administration in the Fentora label”.

Second-quarter sales of Fentora, which was launched in October, reached $68 million but the warning letter could “lead to more cautious prescribing of the product,'' according to a research note from analyst Annabel Samimy at UBS Investment Research. That concern would appear to be shared by investors but CIBC World Markets said the share price fall was an "overreaction," and should not lead to physicians prescribing the drug less.