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
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.