5 Minutes on anticoagulation
The many interactions of warfarin
by Gregory Curnew, MD
Avoid these common pitfalls on the road to a healthy INR
Vol.18, No.09, October 2010

Avoid alcohol

Daily drinking increases the metabolism of warfarin and decreases PT/INR. But acute ethanol ingestion (binge drinking) decreases the metabolism of warfarin and increases PT/INR. Neither is safe.


Vitamin E may potentiate warfarin’s effect. Cranberry juice is particularly noted for increasing warfarin’s effect.

The anticoagulant effects of warfarin may be decreased if taken with foods rich in vitamin K. Patients should avoid eating very large amounts of liver, green tea, and leafy green vegetables (alfalfa, asparagus, broccoli, Brussels sprouts, cabbage, cauliflower, green teas, kale, lettuce, spinach, turnip greens and watercress). The key is not to avoid vitamin K; instead try to maintain a consistent intake of 70-140 mcg/day. As a general rule, one should refrain from changing dietary habits when a stable target INR has been reached on warfarin.


Ginseng (American), coenzyme Q10, and St. John’s wort may decrease warfarin levels and effects.

All of these herbs have their own anti-clotting effect and should be avoided to minimize risk of bleeding: anise, bilberry, bladderwrack, bromelain, cat’s claw, celery, chamomile, coleus, cordyceps, dong quai, evening primrose oil, fenugreek, feverfew, garlic, ginger, ginkgo biloba, ginseng (except American), glucosamine, grapeseed, green tea, guggul, horse chestnut seed, horseradish, licorice, omega-3-acids, prickly ash, red clover, reishi, sweet clover, turmeric and white willow.

Special populations

Patients with genomic variants in CYP2C9 and/or VKORC1 genes may be at higher bleeding risk. The relevant *2 or *3 alleles of CYP2C9 are found in 18% of North American Caucasians and 6% of North American Blacks.

Extra caution is also needed in the anticoagulation of ovulating women, as they risk developing ovarian hemorrhage at the time of ovulation.

The elderly may be more than usually sensitive to anticoagulant therapy. Older patients, whose diets are often quite poor in fruit and vegetables, can have a dramatic increase in INR when starting warfarin therapy, especially if they’ve been taking antibiotics that changed their GI micro flora. A steady, consistent diet is the best policy.

The lists above are perforce incomplete, since it would take many pages to list all of the drugs and herbs that interact in some way with warfarin. It’s essential to check the potential for interaction in an anticoagulated patient whenever in doubt. Some drugs, like allopurinol, would ideally be discontinued during warfarin therapy. But many of the interactions listed here don’t preclude the agent or food being taken with warfarin. Rather they indicate the need for caution and monitoring (in drugs) and moderation (in food). They also serve as clues which, taken together with a good history, may explain why a patient’s INR is not where it should be.

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