Fri Feb 14, 5:38 PM ET

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By Maggie Fox

WASHINGTON (Reuters) - People who take baby aspirin or coated aspirin to try to prevent heart attacks or strokes may not be getting the benefits they expect, US researchers said on Friday.

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A study of more than 250 people showed that most of those who took low-dose or coated aspirin in fact saw no reduced blood clotting, the researchers told a meeting of the American Stroke Association in Phoenix.

Full-sized, uncoated aspirin seemed more effective, according to a measurement of the blood's stickiness called platelet function, said Dr. Mark Alberts, director of the Stroke Program at Northwestern Memorial Hospital in Chicago.

"More than 50% of patients who got coated or low dose aspirin seemed to have normal platelet function," Alberts said in a telephone interview. "This is remarkable."

He said his study would help explain why aspirin does not seem to work for many people. "About half the patients who have a heart attack or stroke are taking aspirin at the time they have a heart attack or stroke," he said.

Several studies have shown that aspirin can reduce blood clotting, lowering the risk of heart attack and stroke. Stroke is the third leading cause of death in the United States, affecting 750,000 people a year and killing 165,000.

But aspirin can be dangerous. It and related drugs are blamed for thousands of deaths every year, so doctors want patients to take the lowest-possible dose that is effective.

Alberts and colleagues tested 126 patients prescribed aspirin after having strokes or clogged arteries to the brain.

LOW-DOSE ASPIRIN WORK IN FEWER THAN HALF

The patients were taking various doses and formulations of aspirin. Alberts found that 56% of those taking 81 milligram "baby" aspirin had no changes in blood clotting.

But 72% of patients taking 325 mg aspirin pills had measurable effects.

He found 65% of patients taking coated aspirin--no matter what the strength--had no reduced clotting, while 75% of patients taking uncoated aspirin did have reduced clotting.

Alberts stressed that no one should be taking aspirin to prevent heart disease or stroke without first visiting a doctor. But he said his study suggests that doctors may want to take the time to check to see whether aspirin is working in a patient, perhaps using the platelet test.

The findings have implications for more than patients.

Bayer this week asked the US Food and Drug Administration (news - web sites) to allow it to market aspirin to prevent a first heart attack or stroke, and it has launched a specially packaged 81 mg coated aspirin product.

In 2002 Bayer sued rival Johnson & Johnson over Johnson's advertising for its St. Joseph brand of aspirin, which comes in a "baby" dose. Bayer argued that larger doses of aspirin were more effective than the lower doses.

Dr. Allen Heller, vice president and head of global research and development for Bayer, said he would have to study the findings further but also stressed that no one take aspirin for preventing heart disease without a doctor's advice. But he said Bayer had little guidance for doctors.

"We don't tell doctors what dose to prescribe," he said.

Aspirin is not approved for children or babies because it can cause a fatal brain condition called Reye syndrome.

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