A six-year study of more than 5,000 British doctors published Friday found no evidence that men who take aspirin daily can reduce their risk of heart attacks.
The finding appears to contradict a study of about 22,000 U.S. doctors reported this week that suggests aspirin taken every other day can cut the risk of heart attack by as much as 47%.
But because the British study involved fewer participants, the British researchers conceded that the U.S. study might carry greater weight.
Both groups of researchers agreed that the combined results of the two studies suggest that middle-aged men who take aspirin could reduce the risk of nonfatal heart attacks by about one-third.
Epidemiologist Richard Doll of Oxford University, who took part in the British study, said he remains convinced that aspirin can prevent heart attacks, despite the study's results.
"I think we had bad luck," he said. "If you take all the evidence, I have no question, no doubt in my mind that taking aspirin daily can reduce the risk of heart attack."
Earlier in the week, Dr. Arnold S. Relman, editor of the New England Journal of Medicine, noted that the British study had a wide margin for error because of the smaller number of physicians studied.
In an editorial that accompanied the U.S. report, which appeared in his journal, Relman added, "The smaller number of subjects as well as the larger dose in the British trial may well account for the difference between its results and those reported in this issue."
The U.S. study involved 22,071 male physicians over a five-year period. Of the 11,034 who took one regular-strength (325 milligram) aspirin every other day, 184 had heart attacks or strokes. The other 11,037 doctors took a placebo, and 259 had heart attacks. Eleven of the heart attacks and strokes in the aspirin group were fatal, compared to 20 in the non-aspirin group.
The British study, published in the weekly British Medical Journal, involved 5,139 doctors over a six-year period. Among the 3,429 who took one extra-strength (500 milligram) aspirin daily, 148 died from heart attack or stroke. Among the 1,710 who did not take aspirin, 79 died of heart attack or stroke.
'Carries More Weight'
The British researchers pointed out that the U.S. study found about three times as many nonfatal heart attacks as the British study. "So the positive result from the United States carries more weight than the null result from the United Kingdom," they wrote.
Both groups found a slight increase in the incidence of strokes among the group taking aspirin.
Heart attacks occur when a blood clot blocks vessels that supply blood to the heart. Aspirin has long been known to impair the clotting ability of blood, and researchers reasoned that its use could reduce the risk of heart attacks by making such clot formation less likely.