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Background :Regorafenib is an oral multi-kinase inhibitor warfarin is an anticoagulant used for the prevention and treatment of venous thrombosis, atrial fibrillation, and pulmonary embolism. Since warfarin is metabolized by CYP2C9 and regorafenib inhibits CYP2C9, concurrent use of regorafenib and warfarin increases the serum concentration of warfarin, which leads to elevation of international normalized ratio (INR).We report two cases of increased INR from concurrent use of warfarin and regorafenib by reviewing electric medical records respectively. Case 1. An age 66 man with pulmonary embolism received warfarin 23 mg/week, and his target INR was 2.0-3.0. He started regorafenib for rectal cancer with pulmonary and brain metastasis, and then INR increased from 1.74 to 5.71 at day 29 of the regorafenib 1st cycle. After