Aspirin prevents blood platelets from clumping together and forming clots that could cause a heart attack or stroke. After a heart attack or stroke, an individual is at greatly increased risk for another. Aspirin has been proven to reduce this risk by 20%. Yet a survey appearing in the Jan. 17, 2012, New England Journal of Medicine found that doctors prescribe aspirin or another antiplatelet agent to fewer than half of patients who could benefit from it. If you have had a heart attack or stroke and have not been told to take daily aspirin, ask your doctor why.