Aspirin is known to prevent heart attacks and strokes.
But a new study in the journal Circulation shows the coating may mask aspirin's heart benefits.
The study was partly funded aspirin-maker Bayer. It says doctors may end up prescribing prescription drugs instead.
Four-hundred healthy people participated in the study.
Researchers did not find a single case of true aspirin resistance. Instead, they claim the coating on the aspirin interfered with the way the drug entered the body, making it appear in tests that the drug was not working.