When taking Coumadin, it is important to avoid consuming large amounts of foods rich in vitamin K, such as spinach, that interfere with the ability of the medication to thin the blood. Cranberries and alcohol are blood-thinners to avoid, according to Mayo Clinic. Warfarin is the generic name for Coumadin.
Leafy greens, Brussels sprouts and broccoli are all sources of vitamin K, according to MedlinePlus. The body uses this vitamin in the blood-clotting process. Doctors sometimes use vitamin K for patients who take too much blood-thinning medication to reverse the effects and stop bleeding.
The green vegetables that contain vitamin K also contain other nutrients that are a part of a healthy diet. Most people who take warfarin can eat small amounts of these vegetables, reports Mayo Clinic. However, large quantities of the vegetables provide enough of the vitamin to reverse the effects of the medication and allow clots to form. Drinking large quantities of green tea provides a similar effect.
Alcohol and cranberries increase the effects of warfarin. Drinking alcohol or cranberry juice in large quantities causes bleeding problems for some patients who take this medication. Nutritional supplements also have the potential for affecting the way warfarin works in the body, so Mayo Clinic recommends consulting a doctor before beginning the use of supplements.