People at high risk for developing blood clots often take warfarin sodium, a blood thinner, daily. If you take warfarin, sold commercially as Coumadin, you must have frequent blood tests to check the time it takes for your blood to clot. Warfarin must be carefully calibrated so that your blood clots less than normal, but doesn't become so thin that you bleed too much. A number of foods contain large amounts of vitamin K, which causes the blood to clot. These foods should be limited because they can decrease the effectiveness of warfarin. Other foods increase the potency of warfarin.
Vegetables
A number of green leafy vegetables high in vitamin K can increase your blood's clotting ability, which works against the effect of warfarin. You do need some vitamin K in your diet, around 120 micrograms for men and 90 mcg daily for women. Vegetables containing more than 200 mg of vitamin K that should be avoided completely include kale, spinach, sauerkraut, broccoli, spring onions or scallions, Brussels sprouts and turnip, beet, dandelion, collard and mustard greens. A serving of parsley, lettuce or asparagus also contains more than the minimum daily vitamin K dose.
Alcohol
Alcohol affects warfarin's actions in several ways. Acute alcohol consumption increases warfarin's effectiveness, increasing your chances of bleeding too much. Chronic alcohol consumption has the opposite effect. Drinking too much on a regular basis decreases warfarin's availability in the body, which decreases the drug's effectiveness, according to the National Institute on Alcohol Abuse and Alcoholism.
Green Tea
Green tea, like many vegetables, contains vitamin K and can decrease warfarin's effectiveness. For this reason, limit green tea consumption while taking warfarin.
Cranberries
Cranberries, unlike other foods, potentiate, or increase the effectiveness of warfarin, which increases your risk of bleeding excessively from cuts or bruises. Spontaneous bleeding can also occur if you take in large amounts of cranberry juice or foods made from cranberries while taking warfarin.
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