Aspirin and clopidogrel (Plavix) are drugs that block the ability of platelets to stick together and form clots. Because the drugs prevent clotting, they may also cause bleeding. Doctors often prescribe daily aspirin to prevent future blood clots in people who have had or are at high risk for heart attacks and strokes. In these groups, the benefits of preventing clots outweigh the risk for bleeding. However, different doses of aspirin may have different benefits and risks. For example, lower aspirin doses may cause less bleeding but may be less effective at preventing clots. Also, higher aspirin doses may be more effective at preventing clots but may cause more bleeding.