Updated from 3:28 p.m. EDT

The New England Journal of Medicine

published a correction of a major research article, adding more fuel to the fiery debate about Vioxx, the arthritis drug withdrawn by

Merck

(MRK) - Get Report

in September 2004.

The correction, whose details were released Monday, may give ammunition to critics who say Merck has downplayed the point at which the potential risks of Vioxx heighten. After the markets had closed, Merck said it stands by the study's original conclusions.

When Merck pulled the drug from the market, it said company-sponsored clinical research showed Vioxx patients had a higher cardiovascular risk than those receiving a placebo. Merck emphasized that the risk was evident only after patients took Vioxx for more than 18 months.

The research, which was published in the

NEJM

in March 2005, plays an important role in Merck's defense against Vioxx-related lawsuits.

Merck is facing at least 11,500 personal-injury suits in the U.S., as well as 190 would-be class-action lawsuits claiming personal injury or financial losses.

"The scientific debate surrounding the

NEJM

study in no way changes our commitment to defending the Vioxx litigation on a case-by-case basis," said Kenneth Frazier, senior vice president and general counsel of Merck. "The individual facts of each Vioxx case are very important."

The

NEJM

correction took aim at the 18-month claim. "Statements regarding an increase in risk after 18 months should be removed" from the article, says the unsigned correction posted Monday on the

NEJM's

Web site. The correction will be published in the July 13 edition.

Originally, the article said "the increased relative risk

of cardiovascular damage became apparent after 18 months of treatment." The original article also said the risk rates between the placebo group and Vioxx group were similar during the first 18 months of a clinical trial nicknamed Approve. The

NEJM

says that statement should be deleted, too.

"As we have said previously and reaffirm today, this correction did not change the data in the

Vioxx study or its results," said Peter S. Kim, president of Merck Research Laboratories.

"It is important to understand that the correction centered on the description of a single statistical method," Kim said. "Merck used a number of analytical and graphical methods to evaluate whether the relative risk changed over time. Taken together and consistent with good statistical practice, these methods confirm that the relative risk changes over time."

Merck's stock closed Monday at $35.02, gaining 8 cents on lighter-than-average trading.

Four weeks ago, the company alerted the publication and government regulators that the

NEJM

article would

require a correction because of a revision in how it measured patients' responses over time.

On May 30, Merck said it didn't believe the correction would alter the study's, or the company's, conclusions about the Approve test. Merck also said it would hire an independent statistical expert to review data and analyses from the Approve study.

"Clearly, an in-depth analysis of the extended experience of patients in the Approve trial is indicated, and it is under way," say two of the study's authors, Dr. Robert S. Bresalier of the University of Texas MD Anderson Cancer Center and Dr. John A. Baron of the Dartmouth Medical School, in a letter posted on the

NEJM

Web site. "Until that is completed and a formal report is peer reviewed, speculations regarding what will be found are premature and may be misleading."