Prasugrel is a next-generation thienopyridine anti-platelet agent that has been approved in over 80 countries for patients with acute coronary syndrome (ACS) undergoing PCI. Prasugrel provides more prompt, potent, and consistent platelet inhibition than clopidogrel, and the effects of prasugrel are not influenced by the presence of CYP2C19 polymorphisms . The efficacy and safety of prasugrel were confirmed in Japanese patients in two phase III studies [8, 9]. Based on these results, prasugrel was approved in Japan in March 2014 for ACS [including unstable angina, non-ST-segment elevation myocardial infarction (NSTEMI), and ST-segment elevation myocardial infarction (STEMI)], stable angina, and old myocardial infarction that requires PCI. The approved doses of prasugrel, which are exclusive for Japanese patients, are 20 mg as the initial loading dose (LD) and 3.75 mg/day as the maintenance dose (MD), which are lower than those used in Western countries (LD/MD: 60/10 mg/day). Given the higher average age and lower body weight of Japanese patients compared with Western patients, we considered that lower doses of prasugrel may effectively lower the risk of bleeding during dual anti-platelet therapy in Japanese patients while maintaining more consistent platelet inhibition than clopidogrel.