Adverse reactions to warfarin are mainly bleeding complications. Approximately 8% of patients taking warfarin experience bleeding adverse reactions each year, of which 1% are classified as serious bleeding, including intracranial bleeding and retroperitoneal bleeding, as well as 0.25% as fatal bleeding, especially untreated hypertension that may lead to intracranial bleeding. Therefore, it is important to monitor the ISI regularly during warfarin administration. An ISI significantly above the target range increases the likelihood of bleeding complications, and of course bleeding may occur in those within the target range. Warfarin is used clinically for the prevention and treatment of deep vein thrombosis and pulmonary embolism, for the prevention of thromboembolic complications following myocardial infarction, and for the prevention of thromboembolism following atrial fibrillation, heart valve disease, or prosthetic valve replacement.