THURSDAY, May 9 (HealthDayNews -- They've been prescribed by doctors for years to reduce risks of first or second heart attack in patients with cardiovascular disease, but a new study is raising concerns that low-dose, coated aspirin might not be as potent as once thought.
"At low doses of 75 milligrams, a significant percentage of people are not receiving sufficient aspirin" to lower their cardiac risk, study lead author Dermot Cox, a pharmacologist at the Royal College of Surgeons in Dublin, Ireland, said in a statement.
In fact, his team found that a normal-weight adult had a 20 percent chance of receiving inadequate cardiac protection from the coated pills, with that number rising even higher among overweight and obese patients.
The findings were reported Thursday at the American Heart Association's annual conference on Arteriosclerosis, Thrombosis and Vascular Biology, held in San Francisco.
Millions of Americans with either a history of cardiac problems or cardiovascular risk factors are currently on daily low-dose aspirin therapy to lower their chances for heart attack or stroke. Because aspirin is linked to stomach ulcers, most take aspirin in a special "coated" form that resists being dissolved in the stomach but is absorbed in the colon instead.
Most experts assumed that this coating would not hinder the overall absorption of aspirin by the gastrointestinal tract.
However, as a matter of routine, Cox's team was asked by the Irish Medical Board -- Ireland's equivalent of the U.S. Food and Drug Administration -- to test coated aspirin for its "bioavailability," which is required of all over-the-counter drugs sold in Ireland.
In their study involving 23 volunteers, the Irish researchers measured blood levels of a chemical called thromboxane as an indicator of aspirin's absorption and its effectiveness in reducing the formation of dangerous blood clots. The lower the level of thromboxane, the better aspirin was working.
They found that plain, uncoated, low-dose (75 mg) aspirin reduced levels of thromboxane to a healthy 0.28 nanograms per milliliter (ng/ml) of blood. In contrast, Caprin, an equal-strength brand of coated aspirin, lowered thromboxane to just 2.24 ng/ml. Two other brands of coated aspirin, Nu-Seals and Protec, reduced blood levels of thromboxane to ineffective levels of 2.75 ng/ml and 5.5 ng/ml, respectively, the study said.
"I think this is an astounding surprise," said Dr. Richard Stein, a spokesman for the American Heart Association and associate chair of medicine at Beth Israel Hospital in New York City. "I had always made the assumption that the [coated] pill dissolved and was absorbed," he said.
Although the study was conducted using commercial brands of aspirin available in Ireland, Stein said the coatings are formulated much the same everywhere, "so I would imagine that the data in the U.S. with these would not be much different."
"I would be surprised if this didn't become an issue for us to look at," he said. "We know that some patients don't show an adequate effect of aspirin, and we've called them 'aspirin resistant' patients. But I think that some of that will be explained by this."
Both Cox and Stein agree that the reduced absorption of daily coated aspirin may be even more of a problem for the overweight and obese, since they require more aspirin than normal-weight patients to achieve the same heart-healthy goals.
Although the study is preliminary, Stein believes it could change standard practice. "I think it's reasonable at this point for physicians using low-dose aspirin -- 75 or 81 mg aspirin -- to think in terms of using the uncoated form, especially for patients who are heavier," he said.
And more study is definitely needed, he added. "We need to see a major study on this in the U.S. now, because it's a product that we're using for primary prevention," Stein explained. "It's probably the most commonly used product to prevent heart attacks in men and women in the U.S. It's given with a small risk each time we use it, and I think we should make sure we're getting the best efficacy from it."