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Confusion also swirls around vitamin K and warfarin. Vitamin K is found in many foods, including leafy green vegetables, such as kale and spinach. Dr. Cho notes that too much vitamin K may reduce the effectiveness of warfarin. However, the foods rich in vitamin K are also part of a heart-healthy diet. She suggests that if you have questions about how much vitamin K is safe, or about any aspect of warfarin therapy, you should talk with your doctor.