Anna D. Garrett, PharmD, BCPS, CPP
Published Online: Sunday, April 1, 2007

Green leafy vegetables, which contain vitamin K, are well-known to cause fluctuations in the international normalized ratio (INR) if eaten on an inconsistent basis. Other foods, such as oils, mayonnaise, and cashews, also contain vitamin K and can cause variations if consumed in large amounts. Patients and providers may be less likely to consider beverages as the source of the problem, however, when the INR starts to fluctuate. Vegetable juice contains spinach, parsley, watercress, and lettuce juices, all of which are high in vitamin K. Supplements such as Boost, Ensure, Glucerna, and Slim- Fast contain 20 to 30 mcg of vitamin K per serving and can have a profound effect on the INR if patients begin using these beverages and fail to mention it to their anticoagulation provider.

Other drinks that have received attention in relation to warfarin therapy include green tea and cranberry juice. Green tea leaves are high in vitamin K, but the brewed tea has only a small amount. Cranberry juice was reported to cause an increase in the INR, possibly through an interaction with the CYP450 system, but a review of case reports did not substantiate this link.

Dr. Garrett is a clinical pharmacist practitioner at Cornerstone Health Care in High Point, NC.

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