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(WZZM/CNN) -- Some forms of aspirin may not reduce the risk of heart attacks.

A new study finds a coating used on the pills could reduce or eliminate the drug's effectiveness in preventing heart attacks and strokes.

Researchers at the University of Pennsylvania found people who appeared to be resistant to aspirin may not properly absorb the pill because of its coating.

Aspirin-maker Bayer, which helped fund the study, disagrees with some of the findings.