Aspirin, a miracle drug packed into a tiny white pill, has had its dosage dictated by a century-old manufacturing process. Now researchers have studied the standard doses that 50 million American adults take daily to prevent cardiovascular disease. And they've found that less is better.

Most people who take daily aspirin to prevent heart disease take either 81 milligrams, a standard baby aspirin, or 325 milligrams, a standard adult pill.

"The 325-milligram dose was arrived at because that's what they could fit into a pill," says Dr. Charles Campbell, lead author of the study and director of the cardiac care unit at the University of Kentucky. "The 81 milligram was arrived at because it's one-quarter of the other."

And there's little science behind recommendations to take either the adult or the child's dose to prevent heart disease. Campbell and researchers analyzed 2,415 studies of aspirin to prevent recurrent events in people who had suffered a heart attack or stroke. "We could find no evidence that higher doses were better," he says. "It also looks like higher doses are associated with more bleeding."

For most people, Campbell says, "Chew or swallow a baby aspirin, and that's enough."

The review was published in the May 9 Journal of the American Medical Association.