Coumadin And Supplements
Chester J. Zelasko, Ph.D. | September 26, 2006

Blood thinners such as Coumadin (generic name: warfarin) are being prescribed more and more today for cardiac problems. There are numerous cautions about consuming anything found in foods and dietary supplements that may affect the clotting ability of the blood--it’s one of the most frequently asked questions we get at Better Life. This Newsletter will examine the research to understand the nature of the concern and whether it’s valid in all cases.

What is Coumadin (warfarin)?
Warfarin is an anticoagulant used to prevent blood clots (for the remainder of the paper, I’ll use the generic name, warfarin). It’s method of action is to interfere with vitamin K in the manufacture of blood-coagulation factors. The decreased quantity of clotting factors prevents blood clots from forming that can cause heart attacks, strokes, or pulmonary embolisms; conditions that can cause such blood clots are arrhythmias such as atrial fibrillation, mitral valve prolapse or valve replacement, and previous heart attacks.

As you’ve probably heard, warfarin also is used as a rat poison. Technically, it doesn’t poison the rats--it causes them to bleed to death by preventing their blood from clotting after being bitten by fleas and ticks in the wild. That shows you how important it is to get the dosage correct!

There are numerous medications that cannot be taken when someone is on warfarin. They’re found in the warning labels provided to physicians, and if you take medications prescribed by more than one physician, be sure you let them all know what meds you’re taking so they can check for potential interactions. It’s a good idea to keep a current list in your wallet or purse so you can show it to any new physician you may see or to emergency medical staff.

International Normalized Ratio
To monitor how the warfarin is working, physicians generally use the International Normalized Ratio (INR). Essentially, a small amount of blood is sampled, the time to clot is measured, and then it’s put into a mathematical formula to determine the INR. Regular blood tests to measure INR are necessary to determine the efficacy of treatment. If the blood clots too fast, more medication is required. However, if it clots too slowly, that can cause problems when someone is injured or even gets a cut. Keeping the INR in the desirable range is possible only through repeated tests.

Foods that contain vitamin K and dietary supplements can potentially interfere with the effectiveness of warfarin. Generally, the patient is told to avoid these foods and those supplements. Period. However, many of those recommendations are based on case studies on individual patients, not clinical trials. Recent research has examined some of these recommendations and called them into question. As an example, let’s examine one dietary supplement, ginkgo biloba, and look at the most current research on interactions with warfarin.

Warfarin and Ginkgo Biloba
Patients on warfarin are told to avoid herbs such as ginkgo biloba. The recommendation is based on several case studies where people using warfarin experienced localized bleeding after ginkgo use. The problem with case studies of individual people is that while it can tell you a lot about that individual, it may tell you next to nothing about how others will respond--that’s the reason for clinical trials. That’s also the conclusion of researchers who completed a review of the evidence on ginkgo, garlic, ginseng, and ginger (1). Fortunately, some clinical trials have been completed.

Researchers examined the effect of ginkgo biloba on patients undergoing long-term warfarin therapy. All patients had stable INRs within desirable ranges (2). The subjects were given 100 mg ginkgo biloba, 100 mg coenzyme Q10, or a placebo for four weeks. The researchers concluded that neither coQ10 nor ginkgo biloba had any effect on patients using warfarin, based on stable INR readings throughout the experimental period.

In another study, researchers examined the effects of ginkgo biloba and ginger on healthy subjects given warfarin (3). At recommended doses, they concluded that neither herbal preparation significantly affects clotting status as measured by INR, the way the medication works in the body, or the way it’s excreted from the body.

Researchers are now examining possible interactions between ginkgo biloba and CYP2C9, an isozyme responsible for metabolizing warfarin (4,5). To date, none have been found.

A Prudent Course of Action
Where does that leave you besides conflicted between what you’ve just read and what your doctor told you? You’re in the best possible position because now you can understand where the cautions came from--case studies--and that research has demonstrated that those results were most likely anomalies, not typical responses; they applied to that particular person, but not people in general.

Does that mean you can take any herbal supplement you want without being prudent? NO! But here’s what you can do.

First, discuss any supplements or dietary changes with your physician. The best way to implement a change is to begin it one week before your next blood test and do it consistently. That way, your body can adapt to the change and any modifications in INR can be noted by your physician and your warfarin level adjusted.

Second, if you decide to make a change such as eating broccoli or a salad most days, or taking a supplement such as omega-3 fatty acids or ginkgo biloba, you must do that consistently. The reason is simple: your physician can adapt the warfarin dosage to whatever you do on a regular basis.

Third, note any changes in bleeding times or slow healing. While the research in clinical trials hasn’t demonstrated any problems, the people in the case studies did have issues. The supplement might or might not have been the cause of the problem--we don’t know. But you must be observant.

The key is to be informed, be consistent, and work with your physician in designing a lifestyle that will help you attain good health. That’s the better life way.

  1. Vaes LP, Chyka PA. Interactions of warfarin with garlic, ginger, ginkgo, or ginseng: nature of the evidence. Ann Pharmacother. 2000;34(12):1478-82.

  2. Engelsen J, et al. Effect of coenzyme q10 and ginkgo biloba on warfarin dosage in patients on long-term warfarin treatment. A randomized, double-blind, placebo-controlled cross-over trial. Ugeskr Laeger. 2003;165(18):1868-71.

  3. Jiang X, et al. Effect of ginkgo and ginger on the pharmacokinetics and pharmacodynamics of warfarin in healthy subjects. Br J Clin Pharmacol. 2005;59(4):425-32.

  4. Mohutsky MA, et al. Ginkgo biloba: evaluation of CYP2C9 drug interactions in vitro and in vivo. Am J Ther. 2006;13(1):24-31.

  5. Greenblatt DJ, et al. Ginkgo biloba does not alter clearance of flurbiprofen, a cytochrome P450-2C9 substrate. J Clin Pharmacol. 2006;46(2):214-21.
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