Clopidogrel, in combination with aspirin, is currently the recommended treatment for secondary prevention of ischemic events in high-risk patients and for prevention of coronary artery stent thrombosis. Patients receiving aspirin and clopidogrel are frequently treated with proton pump inhibitors, such as omeprazole or pantoprazole, in order to prevent the risk of gastrointestinal bleeding, accorded to guidelines. An interaction between proton pump inhibitors and clopidogrel has been suggested, which may lead to a decrease of clopidogrel effects. It remains unclear whether this interaction between PPIs and clopidogrel might be a class effect or if this may be affected by timing regimen. The objectives of this two-phase investigation are: 1. to compare clopidogrel platelet inhibitory effects when taken at the same time versus separated at least 8 hours from omeprazole administration. 2. to compare clopidogrel-induced inhibitory effects when taken at the same time versus staggered at least 8 hours from pantoprazole administration. The clopidogrel dose will be a 600mg loading dose followed by a 75mg daily maintenance dose, starting the next day for 7 days. Omeprazole will be used at a daily dose of 40mg and pantoprazole at 80mg.