Clopidogrel is a thienopyridine antiplatelet agent, which inhibits the purinergic P2RY12 receptor on platelets and prevents their aggregation. It is commonly used in patients with acute coronary syndromes (ACS) undergoing percutaneous coronary intervention (PCI). CYP2C19 is one of the principal enzymes involved in the bioactivation of clopidogrel from the pro-drug to its active metabolite. The most common loss of function (LOF) allele is *2 (c.681G>A; rs4244285), with frequencies of ~15% in Caucasians and Africans and 29-35% in Asians. A large meta-analysis demonstrated that CYP2C19*2 carriers treated with clopidogrel have a higher risk for major adverse cardiac events compared to noncarriers.Therefore, clopidogrel is less effective in patients who are CYP2C19 poor metabolizers and alternative therapy is recommended. A newer-generation thienopyridine, prasugrel, was found to be associated with a reduction in major adverse cardiac events (death, myocardial infarction, stroke) compared to clopidogrel, but with an increased risk of fatal and major bleeding events.