Warfarin, also known under the brand names of Coumadin, Jantoven, Marevan and Waran, is an oral anticoagulant used worldwide for the treatment and prevention of thromboembolic disease. However, warfarin therapy can be difficult to manage because there is a wide variability in patient response and the drug has a narrow therapeutic index. Taking too small a dose can lead to a blood clot while too much can cause life-threatening bleeding.