A prestigious medical journal on Thursday said Merck & Co. withheld information about the dangers of arthritis drug Vioxx in a key study, an alleged lapse that analysts said could hurt Merck as it defends itself against Vioxx-related lawsuits.

The New England Journal of Medicine said it had determined that Merck deleted data about three heart attacks among Vioxx users, and other relevant data, prior to submitting its analysis from the so-called Vigor trial to the Journal in 2000.

The trial compared the safety of Vioxx with naproxen, a widely used rheumatoid arthritis drug.

“The evidence has raised questions about the integrity of the data on adverse cardiovascular events in the article and about some of the article’s conclusions,” the Journal said in a statement on its Web site.

In response, Merck said it promptly and appropriately disclosed the results of the study, correctly stated possible risks of Vioxx and extensively disclosed the Vigor data to the medical community.

Vioxx was withdrawn in September 2004 after being shown to double the risk of heart attack and stroke in patients taking it for over 18 months. More than 6,000 lawsuits have been filed against Merck in the United States, alleging Vioxx caused heart attacks and deaths.

The latest case, brought by a woman whose husband died after taking Vioxx for less than a month, went to a federal jury in Dallas on Thursday.

“If I was one of the attorneys suing Merck, I would be very excited about this,” said Natexis Bleichroeder analyst Jon LeCroy,” referring to the Journal’s allegations. “It would imply that the company actively omitted data in a public forum, trying to make their product look better.”

Merck shares fell 2 percent on the New York Stock Exchange on the news and another 4.7 percent in after-hours trade.

The Journal said it had made the discovery of the alleged deletions as part of preparations for the recent deposition of the executive editor of the Journal in connection with Vioxx-related litigation.

The Vigor study appeared in the Journal in November 2000, soon after Vioxx was launched in the United States to great fanfare as a painkiller that is much gentler on the stomach than conventional treatments like aspirin and naproxen.

The Journal said Merck had submitted its manuscript both on paper and on a computer diskette, but that the Journal’s pre-publication review and editing of the story were completely on the printed version of the manuscript.

The Journal said it did not review the diskette until October 2004, several days after Vioxx was withdrawn.

“In reviewing the diskette, we learned that data on cardiovascular events had been deleted from the manuscript before it was submitted,” the Journal said.

Three heart attacks among the Vioxx group were not included in the printed manuscript, the Journal said.

It said a memorandum dated July 5, 2000 recently made available to the Journal indicates at least two authors of the Vigor manuscript knew about the unmentioned heart attacks at least two weeks before they submitted the first of two revisions to the manuscript and 4-1/2 months before the article was published.

The additional heart attacks became known after the publication’s “cutoff” date for data to be analyzed and were therefore not reported in the Journal article, Merck said.

Instead, Merck said it reported them to U.S. regulators in 2000 and included them in subsequent press releases.

“We also note that these additional events did not materially change any of the conclusions of the article,” Merck said in its statement.

The Journal, however, contended the omissions of the additional heart attacks resulted in an “understatement” of the heart risks of Vioxx, and asked that the authors of the study “submit a correction to the Journal.”

Even without inclusion of those three heart attacks, the Vigor article showed that rheumatoid arthritis patients taking Vioxx had about five times the incidence of heart attacks as those taking naproxen.

Merck later defended the heart-safety of Vioxx, theorizing that Vioxx had done no harm but that naproxen had somehow protected patients from heart attacks.

Vioxx remained on the market and generated annual sales of $2.5 billion by 2003, bolstered by splashy Merck ads.

A New Jersey jury just last month found Vioxx was not responsible for the heart attack in a postal worker who had taken the drug for two months.

However, a Texas widow won the first case against the company when a jury ruled in August the company was at fault in the death of her husband. That jury awarded her $253 million in compensatory and punitive damages, although that amount is likely to be trimmed to about $26 million because of Texas limitations on damage awards.

Merck stock fell to $28.87 on the Inet electronic brokerage from a close of $29.68 on the New York Stock Exchange.