LOS ANGELES (Reuters) - Merck & Co Inc. is evaluating why its withdrawn painkiller Vioxx raises the chance of heart attack and stroke, but it has not found a mechanism of action, a company official said on Monday.

"There is no clear correlation between high blood pressure and those who had cardiovascular events," said Alise Reicin, vice president of clinical research at Merck Research Laboratories. "The mechanism of action is unknown.... We are continuing to look."

Merck recalled its $2.5-billion-a-year Vioxx last month after twice as many patients taking it for at least 18 months suffered heart attacks and strokes during a 2,600-patient trial of the drug's ability to curb colon polyps.

During the three-year study, 45 patients on Vioxx suffered cardiovascular events, compared with 25 on placebo, Reicin said at a meeting of the American College of Rheumatology in San Antonio.

She said 37 percent of patients going into the trial were at high risk of heart problems -- meaning they had a history of problems or at least two risk factors for heart disease.

Reicin said Merck is still collecting data on the impact of Vioxx on the formation of colon polyps and expects to have those results by early next year.

Two other cancer-related studies of the drug -- one in men at risk of prostate cancer and another related to preventing colon cancer -- have also been halted.

Reicin said Merck expects to have follow-up data more than a year from now on how patients in the original colon polyp trial have fared after being taken off Vioxx.

Vioxx is part of a class of drugs that work by blocking the COX-2 enzyme that causes inflammation.

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