Pfizer told the US Food and Drug Administration (FDA) advisers on Wednesday it objected to assertions that all prescription pain relievers similar to Merck's Vioxx could damage the heart.

Merck earlier told the panel meeting that other medicines like Vioxx - known as COX-2 inhibitors - also raise cardiovascular risks.

Pfizer responded that the heart attack and stroke risks reported with Vioxx appeared "distinct" among the COX-2 class. Pfizer sells COX-2 drugs Celebrex and Bextra.

The risks "may be different in compounds within the class", said Kenneth Verburg, a Pfizer vice-president.

Merck voluntarily withdrew Vioxx in September after a study showed the drug doubled the risk of heart attack and stroke in patients who took it for at least 18 months.

Questions raised

Since Merck's Vioxx recall, questions have arisen about Pfizer's Celebrex and Bextra, as well as non-prescription drugs such as naproxen.

Studies "strongly suggest" the entire class of COX-2 inhibitors elevate the chances of cardiovascular problems, Dr Ned Braunstein, senior director of Merck Research Laboratories, said.

Even so, Merck is still pursuing US approval of another COX-2 inhibitor called Arcoxia.

COX-2 inhibitors were designed to ease pain as well as older, non-prescription drugs known as non-steroidal anti-inflammatory drugs or NSAIDs, while being easier on the stomach.

Both Pfizer and Merck stressed that little is known about how older NSAIDs such as ibuprofen and naproxen affect the heart, which makes it difficult to tell if the COX-2 inhibitors are more or less dangerous with regard to heart-related risks.

Trial findings

Last December, Pfizer announced Celebrex more than doubled the risk of heart attack in a large cancer-prevention trial, but the company has described that finding as an anomaly.

Pfizer recently added a warning to Bextra about heart problems in bypass patients, but the pharmaceutical company's officials said overall, the drug appeared no more risky than older NSAIDs.

FDA medical officer James Witter said the data collected so far on Bextra was limited, and it was too soon to draw a firm conclusion on the drug's safety.

"The absence of evidence is not evidence of absence. There is still more we need to know," he said.