A few days are required for full effect to occur, and these effects can last for up to five days. The anticlotting protein C and protein S are also inhibited, but to a lesser degree. Without sufficient active vitamin K 1, clotting factors II, VII, IX, and X have decreased clotting ability. Warfarin decreases blood clotting by blocking vitamin K epoxide reductase, an enzyme that reactivates vitamin K 1. The effects of warfarin may be reversed with phytomenadione (vitamin K 1), fresh frozen plasma, or prothrombin complex concentrate. Many other medications and dietary factors can interact with warfarin, either increasing or decreasing its effectiveness. The effects of warfarin are typically monitored by checking prothrombin time (INR) every one to four weeks. Less common side effects may include areas of tissue damage, and purple toes syndrome. The common side effect, a natural consequence of reduced clotting, is bleeding. It is usually taken by mouth, but may also be administered intravenously. Less commonly, it is used following ST-segment elevation myocardial infarction and orthopedic surgery. While the drug is described as a "blood thinner", it does not reduce viscosity but inhibits coagulation, and is commonly used to prevent blood clots in the circulatory system such as deep vein thrombosis and pulmonary embolism, and to protect against stroke in people who have atrial fibrillation, valvular heart disease, or artificial heart valves.
Warfarin is an anticoagulant used as a medication under several brand names including Coumadin, and as a poison for rats and other pests.
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