Warfarin is a medicine that “thins” the blood. Doctors prescribe it to treat or prevent blood clots. Use of the drug requires testing to see if the blood is thinned too little, too much, or just enough. Blood that is too thin requires withholding or decreasing a warfarin dose. Blood that is not thin enough requires increasing a dose. Once testing shows that a person is receiving the proper dose for the right amount of blood thinness, the same dose can be used in some people indefinitely. Other people require frequent dose adjustments. In theory, people whose blood thinness does not change with a steady dose of warfarin can safely go longer periods between times when their warfarin dose is assessed by their doctors and changed if needed.