Warfarin is a medicine that “thins” the blood. Doctors prescribe warfarin to treat or prevent blood clots. Too much warfarin can thin the blood too much, causing a bleeding event. Treatment is temporarily stopping the warfarin. Vitamin K also helps reverse warfarin's effects. Doctors sometimes prescribe vitamin K to people whose blood is too thin from too much warfarin. However, it is not known whether people who take vitamin K are less likely to have a bleeding event and whether it can reverse warfarin's effects too much, thickening the blood and causing blood clots.