Always check with your doctor and pharmacist about how a specific medication interacts with food. For example, some leafy greens and foods high in vitamin K can affect blood thinners. If you are taking a blood thinner, you don’t need to avoid leafy green vegetables entirely — you just need to be consistent in your intake and not go overboard. Also, grapefruit can affect the absorption and metabolizing of certain drugs. Salt substitutes often replace salt with potassium and can decrease the effectiveness of medications for high blood pressure and congestive heart failure.