[Reinforcement of warfarin action in a patient administered S-1]

A case showing reinforcement of the action of warfarin and potassium in a patient administered S-1 is reported.The patient was a 71-year-old man with left upper gingival cancer.He had ventricular tachycardia (VT), hypertrophic cardiomyopathy, and a cerebellar infarction.He underwent a pacemaker implantation, and was administered warfarin.After the operation, in mid-March 2010, he was administered with S-1 and warfarin. However, the international normalized ratio of prothrombin time (PT-INR) increased to an extremely high level of 5.82, and S-1 and warfarin were stopped. They were re-administered at the end of April, and the PT-INR stabilized to approximately 2.