One nutrient that can lessen warfarin's effectiveness is vitamin K. It's important to be consistent in how much vitamin K you get daily. The adequate intake level of vitamin K for adult men is 120 micrograms (mcg). For adult women, it's 90 mcg. While eating small amounts of foods that are rich in vitamin K shouldn't cause a problem, avoid eating or drinking large amounts of: Kale; Spinach; Brussels sprouts; Parsley