Instructed patient 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. While eating small amounts of foods that are rich in vitamin K should not cause a problem, avoid consuming large amounts of certain foods or drinks, including kale, spinach, brussels sprouts, collards, mustard greens, chard, broccoli, asparagus, green tea. Patient verbalized understanding.