Vitamin K, the blood coagulation vitamin, antagonizes the effect of blood thinners like Warfarin (Coumadin). Thus, if you are on such a blood thinner it is important to keep your vitamin K intake constant and to only eat 1 serving of a high vitamin K food, and no more than 3 servings of a food with moderate amounts of vitamin K. What is most important is that your intake of vitamin K stays consistent.
Since most green vegetables, and "juicing vegetables" are high in vitamin K, it is best not to make juice from them. Juices should be made from low vitamin K fruits, and/or dairy products. Soy products should be avoided, however, juice from other beans and grains is generally ok.
Fruits which affect PT/INR and warfarin should be avoided. These include citrus (grapefruit, oranges, lemons, limes), cranberries, pomegranate, mangoes, and blueberries.
For a list of low vitamin K foods to use, see the article on low vitamin K foods for a warfarin diet. You can also use the nutrient ranking tool to find more foods low in vitamin K. Below are some sample low vitamin K juice recipes.