Seniors who took the painkiller Vioxx faced the highest risk of heart attack during the first two weeks on the drug, according to a large study of patients in Quebec published yesterday in the Canadian Medical Association Journal.

The study raises new questions about the safety of even short-term use of the drug, which was withdrawn from the market by maker Merck & Co. after a 2004 study showed that long-term use increased users’ heart-attack risk. Merck faces roughly 11,500 lawsuits over the drug. A key element of its defense has been the argument that it takes 18 months for Vioxx to raise a patient’s cardiovascular risks.

The Canadian study looked at cardiovascular problems among 9,832 patients over the age of 65 who took Vioxx and a related drug, Pfizer Inc.’s Celebrex, which is still on the market.

Of the 3,947 patients who took Vioxx during the study, 239 had heart attacks. Among those who had heart attacks, 65 cases happened in people who were first-time users of the drug and had filled only one prescription. The risk was highest for people between six and 13 days after their first exposure. Patients’ risk of heart attack did not rise further with extended use, and returned to normal a week after stopping the drug.

Among the 5,885 Celebrex users in the study, there appeared to be a slightly higher incidence of heart attack in the early weeks of treatment, but the increased risk was not “statistically significant,” said the studies lead author, Linda Lvesque, a doctoral candidate at Montreal’s McGill University. Still, “there may be an increased early risk with Celebrex,” said Ms. Lvesque, who is an assistant professor of epidemiology at Queen’s University in Kingston, Ontario. “This needs to be verified in a larger study.”

Gail Cawkwell, senior medical director for Celebrex at Pfizer, said, “It’s going to be very important to consider the safety of Celebrex compared to other arthritis treatments,” and pointed out that last year the Food and Drug Administration said all nonsteroidal anti-inflammatory drugs may increase cardiovascular risks.

The Vioxx findings could support the hypothesis that the drug leads to a higher risk of heart attack by promoting blood clotting in at-risk patients. “If there are people out there who have heart disease, exposure [to Vioxx] should tip the balance shortly after exposure occurs” rather than months later, said David N. Jurlink, a clinical pharmacologist at Sunnybrook Health Sciences Center who was part of a team that conducted studies on Cox-2 inhibitors, such as Vioxx.

In a statement, Merck said observational studies such as this one, which look backward trying to find links between medical events and outcomes, are considered less scientifically rigorous than clinical trials. “It doesn’t change our strategy, and we continue to believe those [short-term] cases have no merit,” said Ted Mayer, co-lead outside defense counsel for Merck.

“From the very beginning, we have said it doesn’t take 18 months,” Andy Birchfield, a plaintiffs’ lawyer in the Vioxx litigation, said. “This strengthens our hand tremendously.”

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