Cephalon Inc has warned doctors about deaths linked to improper use of its cancer pain drug Fentora, U.S. drug regulators said on Thursday, sending the company's shares down 5 percent.

The U.S. Food and Drug Administration posted on its Web site Cephalon's letter to doctors dated September 10, warning of deaths and serious side effects in patients treated with the pain killer.

Cephalon spokeswoman Candace Steele said the company has received reports of three deaths related to inappropriate prescribing of the drug. The deaths occurred during the summer and are most likely due to respiratory failure, she said.

Two deaths were in patients who could not tolerate narcotics and were prescribed Fentora for headache or migraine. One death was associated with improper dosing, Steele said.

Cephalon also received a report of a person who committed suicide while thought to be on the drug, although Fentora was not prescribed for that patient by a physician.

CIBC Markets analyst Bret Holley said he did not expect doctors to stop prescribing Fentora because it is among potent pain-killing drugs that are known to carry similar risks.

"The risk of death is a serious concern with any strong opioid," Holley said in an investor note. "The headline is scarier than any potential impact on sales."

Cephalon said 78,000 prescriptions were written for the drug between October 2006 and September 2007.

An FDA spokeswoman was not available for further comment.

Last year, an investigation by Connecticut's attorney general found that Cephalon illegally promoted some drugs for uses for which they are not approved, including Actiq, Cephalon's older cancer pain medicine.

Cephalon is also one of a host of companies that have been subpoenaed in a U.S. congressional probe into off-label treatments, which occurs when doctors prescribe products for uses other than those officially approved by regulators.

Fentora is approved to treat so-called breakthrough pain, pain experienced despite regular pain medication, in patients with cancer who are already receiving and able to tolerate opioid therapy such as morphine or oxycodone.

The company is seeking approval to expand use of the drug into other pain conditions, such as back pain.

Cephalon shares closed $3.99 lower to $72.55 on Nasdaq, after going as low as $71.86 earlier in Thursday's session.