This is a randomized-controlled clinical trial that will randomize 120 patients undergoing clinically-indicated coronary artery bypass graft surgery to prasugrel at a dose of 10 mg daily or matching placebo for 12 months, starting at the time of hospital dismissal from surgery. The primary goal of the study is to determine whether prasugrel administration will prevent thrombus (clot) formation within a saphenous vein graft at 12 months, as examined by optical coherence tomography.