Good news for Coumadin patients

Dr. Frank Shallenberger, MD

January 3, 2022


Are you taking Coumadin, also known by its generic name, warfarin? If so, you’re not alone.

Most of my new patients, and a large percentage of my readers, take the drug. Coumadin is the most widely prescribed anti-coagulant medication in the world.

This drug blocks the ability of the blood to coagulate and form clots.

If you have atrial fibrillation, blood clots in the legs, pulmonary embolism, or an artificial heart valve, you probably take Coumadin. All of these conditions run a higher risk of clotting.

One of the most aggravating aspects of Coumadin is that there are quite a few factors other than vitamin K that can either decrease or increase its effects.

One of those factors used to be cranberry juice. Now it isn’t.

A 2008 study published in the British Journal of Pharmacology concluded that cranberry juice increases the effect of Coumadin by 30%. So many doctors advised their patients to avoid cranberry juice while on Coumadin. But now, a newer study refutes this idea.

Researchers at the Boston Medical Center studied 30 patients taking Coumadin. All of them were on an established stable dose. Some of them drank eight ounces of cranberry juice every day for two weeks. Others drank a placebo drink that looked and tasted like cranberry juice.

The researchers found that the cranberry juice had no effect at all on the anti-clotting effect of Coumadin. That means Coumadin takers can now safely drink at least eight ounces of cranberry juice every day if you want to. Makes you wonder how these wives tales get around.

But There’s More to This Story

Coumadin works by creating a vitamin K deficiency. Like all vitamins, vitamin K has many beneficial effects, and one of them is to assist in clotting.

When there’s a big enough deficiency of vitamin K, the ability of your blood to clot ceases, bleeding becomes uncontrolled, and you may bleed to death. So taking vitamin K or foods that contain vitamin K will offset the anti-vitamin K activity of Coumadin.

That’s why doctors often tell their patients on Coumadin to avoid foods high in vitamin K. These include broccoli, spinach, Brussels sprouts, kale, asparagus, and lettuce.

But is this necessary? And, in fact, is it healthy?

These foods contain a lot of vitamin K. But they also contain an abundance of other nutrients that are important, and that your body needs. That’s why I disagree with the idea that patients on Coumadin should avoid these foods.

Instead, I tell my patients to eat these foods, as much as they want, but eat them in a consistent way. For example, don’t eat a lot of vitamin K foods one week, and then none the next week.

Eat the same general amount on a regular basis. That way your doctor can adjust your Coumadin dose according to your diet instead of you adjusting your diet according to your Coumadin dose.

What About These Herbs?

The same is supposedly true for many important herbs. St. John’s wort, ginger, garlic, ginseng, gingko biloba, borage oil, and fish oil have all been accused of interfering with Coumadin. But do they really?

In a 2005 study in the Archives of Internal Medicine, the authors reviewed the entire literature dealing with the effects of various medications and herbs on Coumadin. They concluded that, “Most reports are of poor quality and present potentially misleading conclusions.”

For example, in one of the studies that erroneously accused cranberry juice of interfering with Coumadin, the authors concluded that garlic in fact was safe to take.

While foods and herbs are safe, this same 2005 review also concluded that there are many drugs which are not. These drugs increase the effect of Coumadin to the point that dangerously excessive bleeding can occur. The drugs include the following:

• azole antifungal medications — metonidazole, clotrimazole, ketoconazole, and itraconazole
• quinolone antibiotics — Avelox (moxifloxacin), Cipro (ciprofloxacin), and Levaquin (levofloxacin)
• nonsteroidal anti-inflammatory drugs — ibuprofen (Advil), naprosyn (Alieve), and Celebrex (celecoxib)
• SSRIs — Celexa, Prozac, Lexapro, Luvox, Paxil, and Zoloft
• Prolisec (omeprazole)
• lipid-lowering agents — statins, Cordarone (amiodarone)
• fluorouracil (a chemotherapy drug)

If you are taking Coumadin and have been advised to start one of these drugs, make sure you discuss it with your doctor.

The bottom line is that anyone taking Coumadin needs to be consistent with their diet, their medications, and with any supplements or herbs they are taking. Once any of these factors changes, then you need to notify your doctor so that he can modify the Coumadin dose if necessary.


Ansell, J., M. McDonough, Y. Zhao, J.S. Harmatz, and D.J. Greenblatt. “The absence of an interaction between warfarin and cranberry juice: a li> • macrolide antibiotics — randomized, double-blind trial.” J Clin Pharmacol. 2009; 49(7):824-30 (ISSN: 0091-2700).

Mohammed Abdul, M.I., X. Jiang, K.M. Williams, R.O. Day, et al. “Pharmacodynamic interaction of warfarin with cranberry but not with garlic in healthy subjects.” Br J Pharmacol. 2008; 154(8):1691-700 (ISSN: 0007-1188).

Holbrook, A., J. Pereira, R. Labiris, et al. “Systematic Overview of Warfarin and Its Drug and Food Interactions,” Arch Intern Med. 2005;165:1095-1106.

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