The U.S. government disclosed Cephalon's warnings that improper use of its severe pain treatment led to several deaths.
In letters to physicians sent Monday, Cephalon said errors by health care professionals prescribing the drug, Fentora, have had sometimes fatal results. The U.S. Food and Drug Administration posted two such letters to its Web site.
Cephalon spokeswoman Stacey Beckhardt said the company has received three reports of patients dying after physicians improperly prescribed Fentora.
Cephalon is one of several drug companies under investigation by the U.S. Congress for allegedly promoting its drugs for uses not approved by the Food and Drug Administration. Earlier this year, the House Oversight Committee ordered Cephalon to turn over all promotional materials for Fentora and an earlier version of the drug, Actiq.
The Connecticut Attorney General's office is conducting a similar investigation of the company.
Doctors are allowed to prescribe medications for any use they deem appropriate, though companies are only allowed to market products for uses approved by the government.
FDA approved Fentora last year to treat "breakthrough pain," or intense bursts of pain, experienced by cancer patients who are already taking more conventional pain killers.
The company is working with FDA on a new drug label to clarify when and how it should be used.
"We believe it's very important to clarify the information already on the label to make sure medical community is using the medication as safely as possible," Beckhardt said.
In its letter, Cephalon warned physicians not to prescribe the medication for types of pain it is not approved to treat. The letter also stresses that patients never take more than two pills per pain episode and should wait at least four hours before treating another pain episode.
The company said Fentora should only be given to patients who are already on opioids, a form of narcotic commonly used to treat pain.
The company said some of the fatal problems have been caused by physicians hastily switching patients from an older version of the drug, Actiq, which the company also makes, to Fentora.
Beckhardt said some physicians have not been adjusting the dosages of the drugs, as described on the products' label, when switching patients from Actiq to Fentora.
Thomas Weisel Partners analyst Donald Ellis said prescribing errors are common with new drugs and that the news probably will not hurt sales of Fentora.
However, Ellis said the safety concerns could make it tougher for Cephalon to win approval for additional uses for Fentora.
Cephalon plans to ask FDA to approve the drug to treat severe pain that is unrelated to cancer in the fourth quarter.
Shares of Cephalon fell $3.63, or 4.74 percent, Thursday to $72.91 in afternoon trading.