Vioxx maker Merck & Co., as well as a lawyer not involved in Vioxx litigation, cautioned that the study, done after users were stricken, is not definitive and is not as reliable as clinical studies done comparing Vioxx with a placebo.

The study by researchers at McGill University Medical Centre in Montreal found that a "small proportion" of patients using Vioxx had their first heart attack soon after starting on the drug, which Merck pulled from the market in September 2004.

Risk Returned to Normal After Stopping Use

"This risk did not increase with the length of treatment and returned to baseline shortly after treatment was discontinued," according to the study, posted Wednesday on the Web site of the Canadian Medical Association Journal.

"A quarter of individuals in our study who suffered an acute myocardial infarction did so within two weeks of their first Vioxx prescription," said the lead author, Linda Levesque, a McGill doctoral student in epidemiology and biostatistics. "The additional cardiovascular risk from Vioxx actually decreased with longer duration of use, suggesting that the period of highest susceptibility for most people taking Vioxx may occur earlier than previously believed."

The study also found evidence that cardiovascular risk returned to normal within one month of stopping the drug, but acknowledged that more research is needed.

Chris Placitella, a lawyer in Red Bank, New Jersey, handling about 1,000 cases for people who took Vioxx, said he and other plaintiff's lawyers will be looking at the Canadian study.

"It certainly lends support for the plaintiffs' claim that short-term duration use of Vioxx can cause cardiac events. It also will cause us to go back and reevaluate some of the cases we rejected because of very short-term use after we have the opportunity to review the results of the study with our experts," Placitella said.

Many Risk Factors Not Taken into Account

Merck, facing about 11,500 lawsuits over Vioxx, said the study would not change its strategy of taking them case-by-case. The Whitehouse Station, New Jersey-based company has lost half of the six lawsuits that have come to trial, with three juries awarding multimillion dollar verdicts against Merck.

An outside lawyer for the company, Jim Fitzpatrick, said that observational studies such as that done by McGill have "significant limitations."

"The randomized clinical trial data, which is extensive, has shown there is not an increased risk with the short-term use of Vioxx," Fitzpatrick said. "This really doesn't change anything."

A lawyer not involved in Vioxx cases who has defended pharmaceutical companies, Murray S. Levin, said that studies like the McGill effort cannot account for many factors, such as the variety of risk factors for heart attack, including weight and genetic background.

However, that probably will not impede plaintiffs' lawyers from using it, Levin said. "It is one more arrow for the quiver that they can fire off," he said.

The McGill study found that of 3,947 patients 66 and older in Quebec who took Vioxx, 239 had heart attacks.

The study also checked records of 5,885 users of another Cox-2 inhibitor, Celebrex, made by Pfizer Inc., and found 287 people had heart attacks, but determined there was not conclusive evidence of increased risk of heart attack.

Pfizer spokesman Daniel Watts said other studies have found no increased risk for heart attack between those taking Celebrex and other nonsteroidal anti-inflammatory drugs.