A correction published Monday to a key study on withdrawn painkiller Vioxx reveals the risk of heart problems was elevated throughout the time people were on the drug and did not develop only after 18 months of use as the drug's maker, Merck & Co., has contended.

The correction in the New England Journal of Medicine supports many doctors' contention that risks showed up with as little as four months of use.

Heart attacks and strokes occurred more frequently after people had been on the drug for at least 18 months, but the actual harm occurred much earlier, said Dr. Jeffrey Drazen, editor-in-chief of the journal, who worked with the authors to correct their original findings of the APPROVe trial, published in March 2005.

"It's a subtle but very critical point," Drazen said.

Cleveland Clinic cardiologist Dr. Steven Nissen, who has challenged the study's conclusions in the past and did so again in a separate letter also published by the medical journal on Monday, agreed.

"A key legal defense in the liability cases has been the suggestion that there was no risk until patients had taken the drug for 18 months," he said. Now, with the correction, "the authors have removed any claim that there was a delay in risk."

The main authors, Dr. Robert Bresalier of the University of Texas' M.D. Anderson Cancer Center and Dr. John A. Baron of Dartmouth Medical School, could not be reached for comment but acknowledge in a letter released by the journal that their statistical analysis had been flawed.

Officials at Merck could not immediately be reached for comment. The company, which paid for the study, acknowledged the flawed analysis last month but claimed it did not change bottom-line results.

Merck faces more than 11,500 lawsuits over Vioxx, a blockbuster arthritis drug until it was pulled from the market in September 2004.

An ongoing trial in Atlantic City, N.J., is the seventh case to reach trial. Merck has won half of the previous six.

Shares of Whitehouse Station, N.J.-based Merck rose 29 cents to $35.23 in early afternoon trading on the New York Stock Exchange.