Dual antiplatelet therapy with clopidogrel and aspirin has been shown to reduce recurrent cardiac events in patients with acute coronary syndromes or those who have undergone coronary artery stent placement. Clopidogrel, a thienopyridine, is a prodrug that is transformed in vivo to an active metabolite by the cytochrome P450 enzyme system. Due to the increased risk of bleeding in patients on dual antiplatelet therapy, concomitant gastrointestinal ulcer prophylaxis with a proton pump inhibitor (PPI) is frequently prescribed. Data from recent studies show that PPIs, which are extensively metabolized by the cytochrome system, may decrease the antiplatelet activity of clopidogrel. This article reviews the metabolism of various PPIs and existing data regarding the drug-drug interaction between PPIs and clopidogrel.