(CNN) — If you take aspirin, check the label. What a new study says about the effects of coated tablets.

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

The University of Pennsylvania study looked at 400 healthy people to determine why some people develop what doctors believe to be aspirin resistance.

Researchers found resistance to the medication may not be the problem at all.

Instead, the coating may prevent aspirin from absorbing in a way that allows is to do its job.

Bayer, which makes aspirin, helped fund the study, but the company disagrees with some of the findings, saying, when used as directed, their products provide “meaningful benefits.”