Herbs & Blood-Thinning Medications
Chester J. Zelasko, Ph.D. | March 20, 2001

Many people today are taking blood-thinning medications. Some are relatively simple, such as taking an aspirin every day to reduce the risk of heart attack. Other medications are more serious including Coumadin, one form of the drug warfarin sulfate. They both act by thinning the blood to differing degrees to prevent blood clots from forming.

Coumadin is used for many serious disorders such as atrial fibrillation, stroke, artificial heart valves, and is used after a heart attack. Some people take Coumadin for several weeks while others, as is the case with artificial heart valves, must take these drugs for the rest of their lives. While life saving in many cases, blood thinners complicate the lives of people who take them because they must restrict what they eat and what food supplements they take.

For people accustomed to taking food supplements, the question becomes, "What food supplements can I take after being prescribed blood thinners by my physician?" The answer is far from simple. The most conservative answer is to stop taking every vitamin, mineral, and herb. But that doesn't seem to be necessary.

The most important vitamin to control is vitamin K. Because it can affect the dosing of blood thinners, the key is to take a consistent amount of vitamin K all the time. Fluctuations are what physicians really want you to avoid, so they will tell you to avoid foods with high vitamin-K levels, such as spinach, broccoli, and alfalfa, and all food supplements with vitamin K in them. While food intake may vary, if you've always taken a multivitamin-multimineral with vitamin K in it, the dosing of blood thinner you've received is probably adjusted for it, but check with your physician to be sure. And don't add a supplement containing vitamin K without advising your physician.

The recommendations for the rest of the vitamins, herbs, and other supplements are much less clear. After reviewing several papers on drug-supplement interactions (1-2), the data are incomplete. Most recommendations to avoid herbs are based on case studies, which are reports of single individuals who had negative reactions when supplementing their diet. Based on the limited data available, the herbs to definitely avoid when on a blood thinner are garlic, willow bark, and ginkgo biloba—not because the data indicate a problem, but because the theoretical basis dictates caution. Always err on the conservative side—avoid those herbals unless you specifically talk to your physician about them.

The usual recommended amounts of vitamin E (200-400 IUs) and omega-3 fatty acids from fish oil (up to 3 grams) appear to be safe. There are no known concerns about other vitamins and minerals.

So what should you do? The most important thing is to discuss any concerns you have about supplements with your physician. He or she understands the interactions to avoid, so listen carefully. But don't hesitate to recommend the references listed below so your healthcare professional can personally examine the data. Most of all, respect herbal preparations. They can be powerful allies in your quest for good health when used wisely, and powerful foes if misused.


  1. Heck, AM, Dewitt, BA, and Lukes, AL. Am J Health-Syst Pharm 57(13):1221-1227, 2000.
  2. Brinker, Francis. Herb Contraindications and Drug Interactions. Second Edition. Eclectic Medical Publications. Sandy: Oregon. 1998.
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