In the United States, approximately 2.7-6.1 million patients had atrial fibrillation (AF) in 2010, and this number is expected to gradually increase to approximately 5.6 to 12 million by 2050. 1,2) The vitamin K antagonists (VKAs), warfarin and acenocoumarol, are prescribed for the treatment or prophylaxis of embolic AF or deep vein thrombosis (DVT). Warfarin was commonly prescribed for older individuals, those with a high risk of stroke or bleeding, and patients with many comorbidities. 3) These patients are likely to have drugdisorder or drug-drug interactions. It is necessary for patient safety and personal medication therapy to regularly monitor prothrombin time (PT) or the international normalized ratio (INR). VKAs have a narrow therapeutic range and, therefore, levels outside this range increase the risk of thromboem-bolism or bleeding incidences.VKAs are drugs that interact with various medications and foods [4][5][6] and maintenance doses of VKAs are affected by patient age, 7) body weight, 8) clinical conditions, 9,10) and genetic factors. 8,11) According to the Clinical Pharmacogenetics Implementation Consortium (CPIC) guidelines, cytochrome P450 2C9 (CYP2C9), vitamin K epoxide reductase complex subunit 1 (VKORC1), and cytochrome P450 4F2 (CYP4F2) are known to influence the warfarin dosing algorithm. 11) Similarly, the maintenance dose of acenocoumarol is also affected by VKORC1 and CYP2C9 genes. 12) In addition to these well-known genes, recent studies have suggested that the ATP-binding cassette subfamily B member 1 (ABCB1) gene affects the maintenance dose of VKAs; however, the