Warfarin is a medicine that “thins” the blood, making it take longer to coagulate. Doctors prescribe warfarin to treat or prevent several common medical conditions, including blood clots and stroke. Patients taking warfarin need to have blood tests called international normalized ratios (INRs) to check that the amount of warfarin they are taking is thinning their blood an appropriate amount. When the INR is too low (underanticoagulation), clots can occur; when it is too high (overanticoagulation), serious bleeding can occur. When patients become overanticoagulated while taking warfarin, doctors temporarily stop the warfarin. They also sometimes start treatment with vitamin K to speed the reversal of warfarin's effects. It is not known whether giving vitamin K by mouth or by injection brings the INR into the desired range more quickly.