Warfarin (also known under the brand names of Coumadin, Jantoven, Marevan, and Waran) is an anticoagulant medication that is administered orally or, very rarely, by injection. It is used for the prophylaxis of thrombosis and embolism in many disorders. Its activity has to be monitored by frequent blood testing for the international normalized ratio (INR). It is named for the Wisconsin Alumni Research Foundation.
Warfarin is a synthetic derivative of coumarin, a chemical found naturally in many plants, notably woodruff (Galium odoratum, Rubiaceae), and at lower levels in licorice, lavender and various other species. Warfarin was originally developed as a rat poison; however, more modern poisons are much more potent and toxic (e.g., brodifacoum). Warfarin and contemporary rodenticides belong to the same class of drugs (coumarins) and both decrease blood coagulation by interfering with vitamin K metabolism. For this reason, drugs in this class are also referred to as vitamin K antagonists.