Filed at 10:54 a.m. ET

DALLAS (AP) -- A new study suggests that some people who take aspirin to ward off heart attacks may not be getting all the benefits they thought they were.

The study in Tuesday's issue of the journal Circulation found that as many as 75 percent of patients showed some resistance to the blood-thinning effects of aspirin.

Aspirin works by blocking the formation of thromboxane A2, a chemical in the body that makes platelets sticky and promotes blood clotting. Heart attacks are caused by clots.

The study found that taking aspirin did not adequately block thromboxane in some people, making them 3 1/2 times more likely to die of a heart attack than those in whom aspirin works.

Doctors often recommend daily use of aspirin to help prevent heart attacks.

The study was led by Dr. John Eikelboom, a clinical lecturer at the University of Western Australia, Royal Perth Hospital.

The researchers analyzed urine samples of 5,529 heart patients in Canada for a chemical byproduct of thromboxane. The levels of the byproduct varied substantially among the aspirin users but were still lower than they were in patients not taking aspirin at all.

Eikelboom theorized that aspirin did not adequately block thromboxane in some patients because of an underlying genetic mutation.

The study could be used to identify heart patients who need to be treated with additional anti-clotting medications, said Dr. Salim Yusuf, a co-author of the study. Still, he cautioned, more research is needed.

Yusuf said a simple urine test could be created to determine if patients are resistant to aspirin.

Medical experts said those who take aspirin because of heart disease should continue to do so.

``We know that aspirin reduces risk of a new cardiac event by as much as 25 percent,'' said Dr. Rose Marie Robertson, a professor of medicine at Vanderbilt University in Nashville, Tenn. ``The big message is that not enough people are on aspirin.''

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On the Net:

American Heart Association: http://www.americanheart.org