When you take aspirin for aches, pains, and to reduce fever, you know that it is working because you soon feel better. When taking aspirin to prevent a heart attack or stroke, you don’t “feel better” if the aspirin is doing its job, because you can’t feel the effects it has on your platelets.

Platelets are small blood cells circulating in your blood stream that stick together to form a blood clot and stop the bleeding at the site of a wound of injury. Just like a cut in your skin, a ruptured plaque in your arteries can cause platelets to form a clot that if large enough, can cut off the blood flow and cause a heart attack or stroke. Platelets can also form a clot when exposed to ruptured plaque in your arteries.

Aspirin works to prevent heart attacks and strokes by reducing the production of thromboxane, a chemical that makes platelets sticky. When aspirin works as it should, platelets make less thromboxane and are less likely to form a blood clot that could block an artery.