NEWARK, N.J. — An attorney for Vioxx users said on Wednesday said that a new study which suggests the pain killer may cause problems with short-term use may lead to more lawsuits against the pain reliever's maker, Merck & Co.

However, a Merck lawyer, as well as an attorney not involved in the litigation, cautioned that the study, done looking at patient records, 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. Whitehouse Station-based Merck pulled Vioxx from the market in September 2004 after a study showed it doubled patients risk of heart attacks and stokes after 18 months of 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 Red Bank lawyer 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.

Merck, facing about 11,500 lawsuits over Vioxx, said the study would not change its strategy of taking them case-by-case. The 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."

Fitzpatrick said that studies done comparing Vioxx to placebo haven't shown an increased risk with short-term use.

"This really doesn't change anything," said Fitzpatrick.

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

Johns Thomas, a health law professor at Quinnipiac University School of Law, in Hamden, Conn., said the McGill study had a small sample size, but will probably lead to more lawsuits.

"It may change the landscape a little but, since Merck is trying to distinguish the short-term users from the long-term users," Thomas 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 drug, 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.

Merck shares fell 33 cents, or 1 percent, to close at $34.15 on the New York Stock Exchange while Pfizer shares fell 1 cent to finish at $25.17.