The reason for all this testing is that the dose required for safe and effective anticoagulation varies a lot from person to person. Moreover, it varies in the same person from week to week depending on the diet. Because warfarin acts by opposing the body's ability to use Vitamin K to the proteins it needs for normal coagulation, the dose of warfarin changes if a person eats more or less Vitamin K than he or she usually does. Everyone who takes warfarin should get a booklet from his or her doctor listing the amount of Vitamin K in a variety of foods. Foods that contain a particularly large amount of Vitamin K include beef liver, pork liver, green tea, broccoli, chick peas, kale, turnip greens, and soybean products including soybean oil, soybeans themselves, soy milk, and tofu.
More information is available here .
Medicines can also change the effect of warfarin on the body. Everyone who takes warfarin should make sure her or his doctor considers the effect on the warfarin whenever a medication change is made. More information on Coumadin is available here . Considerable research is still going on to determine this dose for warfarin. The problem is difficult because the problems that come with too little warfarin are so bad that it is not ethical to give a dose that has much chance of being too low.