A correction published Monday by a medical journal to a key study on withdrawn painkiller Vioxx reveals the risk of heart problems was elevated throughout the time people took the drug and did not develop only after 18 months of use as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

Legal experts said the correction in the New England Journal of Medicine, one of the world’s premier medical publications, undermines Merck’s credibility and the legal strategy it has been using as it faces thousands of lawsuits over Vioxx, a blockbuster drug that once brought $2.5 billion in annual sales.