Senior citizens who were administered Merck's arthritis medication Vioxx faced the highest risk of heart attack during their first two weeks on the drug, according to a study published in the Canadian Medical Association Journal.

The study, led by researchers at McGill University in Montreal, investigated 9,832 patients over the age of 65 who took Vioxx (rofecoxib)and Pfizer's Celebrex. Of the 3,947 patients who took Vioxx, 239 had heart attacks. Sixty-five of those cases were first-time users of Vioxx and had filled only one prescription of the drug. The heart attack risk was highest, found the study, between the sixth and 13th day of treatment. The risk did not rise with continued use of Vioxx, and actually decreased a week after patients ceased taking the drug.

"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," noted the study. "About one-quarter of current users of rofecoxib who had [a heart attack] experienced this event within a few weeks of receiving their first prescription."

In the study subjects who took Celebrex, there was a slightly higher incidence of heart attack during the first weeks of treatment, but it was "statistically insignificant," according to Linda Levesque, the study's lead author and a doctoral candidate at McGill.

Vioxx was withdrawn in late 2004 amid concerns of increased stroke and heart attack risk. Celebrex is still on the market.

The study's authors suggested that Vioxx may have triggered early-onset cardiac events in particularly at-risk patients -- such as those with heart disease -- who would be more immediately sensitive to the medication. The observed decrease in the risk of heart attack over time, noted the study, "is presumably owing to the depletion of susceptible people." Further investigation of the drug was warranted, the study concluded.