NEW YORK (CNNMoney.com) - A Canadian study involving thousands of Vioxx patients concluded they faced the highest risk of heart attack during the first few weeks of taking Merck's arthritis painkiller.

For Merck (down $0.33 to $34.15, Research), this is the second dose of bad news regarding short-term users of Vioxx in as many weeks. On April 21, in the most recent Vioxx case to reach a verdict, a jury in a Texas border town held Merck liable for the fatal heart attack of a man who had taken Vioxx for less than one month. The jury in Rio Grande City held Merck liable for $32 million in damages, though Texas law caps the damages at $750,000.

Merck faces more than 11,500 lawsuits from former Vioxx users and their families who blame the drug for heart attacks, fatal and non-fatal. Merck pulled the popular drug off the market on Sept. 30, 2004, after a study showed an increased risk of heart attack and stroke in patients who had taken the painkiller for at least 18 months.

Vioxx was a key earner for Merck, with sales totaling $2.5 billion its last full year on the market. Merck's stock price has plunged 24 percent since it withdrew the drug from the market, worrying analysts and investors about the drug giant's future.

Merck has said all along that its drug never killed anybody, and has denied accusations that it deliberately withheld information about Vioxx health risks. In cases filed by plaintiffs who used Vioxx for only short periods of time, Merck's lawyers said there was no proof to back up their claims. The latest findings by the Canadian researchers, coupled with the latest jury verdict from Texas, seem to call into question that legal strategy.

But Merck said that study was based on existing medical records of patients and has less credence than Merck's own studies, which were clinical trials conducted in controlled settings.

"It doesn't change our strategy whatsoever," said Jim Fitzpatrick, an outside counsel for Merck. "It's an observational study, where you're just looking back into a database. That's very different and less reliable scientifically than when you look at clinical trials. The clinical trial data has been consistent in showing that there's no increased health risk in short-term use."

The study, reported in the Canadian Medical Association Journal, is based on the medical records of 3,947 patients over the age of 65 who took Vioxx, and 239 of them had heart attacks.

"In this large, population-based study of elderly people we demonstrated that the cardiovascular risks associated with the use of rofecoxib are more acute that previously recognized," read the report, using the scientific name for Vioxx. "About one-quarter of current users of rofecoxib who had [a myocardial infarction] experienced this event within a few weeks of receiving their first prescription."

The study said that the heart attack risks peak in the first few weeks of taking Vioxx, and then decrease over time. The study also focused on 5,885 patients who took Celebrex, an anti-arthritis painkiller from Pfizer (up $0.09 to $25.27, Research), and 287 of them had heart attacks. Celebrex is a member of the same Cox-inhibitor drug class as Vioxx, but Celebrex was not removed from the market and is still used by patients today. However, in July, 2005, the Food and Drug Administration had Pfizer add a "black box," the most severe type of health warning, to the Celebrex label to reflect risks of heart attacks, strokes and gastrointestinal bleeding. Another Cox-inhibitor from Pfizer, Bextra, was taken off the market in July, 2005, because of health risks.

The Canadian study acknowledged that Celebrex might contribute to heart attack risks, but said the results were inconclusive.

"The short-term use of celecoxib may also be associated with a clinically significant elevation in risk, but, given that statistical confirmation was lacking, further studies will be required to more completely assess this risk," read the study, using the scientific name for Celebrex.

Les Funtleyder, analyst for Miller Tabak, was skeptical of any data compiled from old statistics, instead of controlled studies, because, "You can basically torture statistics to get them to confess. You can find anything if you look hard enough."

But Funtleyder said the study could still pose problems for Merck, because it's another potential weapon in the hands of plaintiff attorneys.

"The science may not seem as strong, but that might not matter with a jury," said Funtleyder. "It doesn't matter how Merck spins it; it's not going to be helpful to them."

Bryan Liang, professor of health law studies at California Western School of Law, said the "retrospective" data from the Canadian study is not as strong as clinical studies, but it comes from a reputable organization and will "add ammunition to the fodder for plaintiff attorneys."

"It's going to really raise issues in juries' minds, for good reasons," said Liang.

Merck has vowed not to settle, but to fight the Vioxx cases one a time. So far, Merck has lost two cases, won two, and received a split verdict in another. Barbara Ryan, analyst for Deutsche Bank North America, believes that $30 billion worth of potential damages are "priced in" to the stock. David Moskowitz of Friedman, Billings, Ramsey has provided the highest damage estimate of $50 billion.

Merck faces its next Vioxx trial on June 5 from two plaintiffs in New Jersey Superior Court in Atlantic City. Merck, the no. 2 U.S. drugmaker, is based in Whitehouse Station, N.J.

Funtleyder does not own shares of Merck stock.

To read about Merck's first quarter earnings, click here.