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.