Ask the Doctor

What supplements should I avoid when taking the drug Warfarin (coumaden)?


Warfarin is an anticoagulant (slows blood clotting) used to prevent and treat people with venous thrombosis (blood clots in the veins) and pulmonary embolism (blood clots in the lungs). Warfarin is also used to treat or prevent dangerous blood clotting in people with atrial fibrillation (an irregular heartbeat) and in some cases to prevent stroke.



Interactions with dietary supplements:


In theory bromelain might potentiate the action of anticoagulants. This theoretical concern has not been substantiated in human research.

Coenzyme Q-10:

Coenzyme Q-10 is structurally similar to vitamin K and has been reported to interfere with warfarin activity. It remains unknown how common or rare this interaction is. Those taking warfarin should only take Q-10 with the guidance of their doctor.



Iron, magnesium and zinc may bind with warfarin potentially decreasing absorption and activity. People on warfarin therapy should take the warfarin and iron/magnesium, zinc containing products at least 2 hours apart.


Vitamin C

Although case reports have suggested that vitamin C might increase the activity of anticoagulants in a potentially dangerous way , this interaction has not been confirmed in research studies.


Vitamin D

In 1975 a single letter to the Journal of the American Medical Association suggested that vitamin D increases activity of anticoagulants and that this interaction could prove to be dangerous. Perhaps because Vitamin D is in most multivitamins taken by millions of people and no other reports of problems have surfaced, most doctors typically do not worry about Vitamin D interacting with anticoagulants.


Vitamin E

An isolated case was reported in 1974 of Vitamin E associated with increased anticoagulation in a person treated with warfarin. More recently double blind randomized trial found warfarin activity unchanged in people treated with warfarin and vitamin E (up to 1200 iu/day) or with placebo. It now appears safe for people taking warfarin to take Vitamin E despite the information often provided by doctors about this dangerous interaction.


Vitamin K

Warfarin slows blood clotting by interfering with vitamin K activity. People taking warfarin should avoid vitamin K-containing supplements unless specifically directed otherwise by their prescribing doctor. Some vegetables (broccoli, Brussels sprouts, kale, parsley, spinach, and others) are high in vitamin K. Eating large quantities of making sudden changes in the amount eaten of these vegetable can interfere with the effectiveness and safety of warfarin therapy.


The important point though is sudden changes. As long as one is consistent with their diet, vegetables and warfarin can coexist. It is sudden fluctuations in the amount of vegetables a person eats that can cause trouble.


Interactions with Herbs:

Asian Ginseng (Panax ginseng)

Ginseng was associated with vaginal bleeding in two case reports and with a decrease in warfarin activity in another case report. Persons taking warfarin should consult a physician with knowledge about botanical medicines if they are considering taking Asian or Siberian ginseng. Again consistency in daily dose is essential.


Dan Shen (Salvia miltiorrhiza)

Dan Shen, a Chinese herb, was associate diwth increased warfarin activity in two cases. Can Shen should only be used under close medical supervision by people taking warfarin. Sage is a relative of Dan shen but is not associated with interactions with warfarin.



Garlic has been shown to help prevent atherosclerosis perhaps by reducing the ability of platelets to stick together. This can result in an increase in the tendency toward bleeding. Standardized extracts have been associated with bleedin in people in rare occasions. Garlic extracts have also been associated with two human cases of increased warfarin activity. The extracts were not shown to be the definite causes of the problem. But until more is known people taking warfarin are cautioned to avoid standardized garlic extracts and to avoid eating more than one clove of garlic a day.



Ginger reduces platelet stickiness in test tubes. There are no reports of interactions with anticoagulant drugs, people should be cautious about mixing ginger with anticoagulant drugs.



Ginkgo extracts may reduce the ability of platelets to stick together, possibly increaseing the tendency to bleeding. Standardized extracts of ginkgo have been associated with two cases of spontaneous bleeding, although the gingko was not shown to be a definite cause of the problem. There is one case where bleeding occurred after the addition of ginkgo. Again it is the sudden changes and lack of consistency that are of greatest concern.


Herbs containing Coumarin derivatives:

Although there are no studies demonstrating interactions with anticoagulants, the following herbs contain coumarin like substances that may interact with warfarin and may cause bleeding. These herbs include:

Don quai


Horse chestnut

Red clover

Sweet clover

Sweet woodruff



Quinine a chemical found in cinchona bark and available as a drug has been reported to increase warfarin activity. People taking warfarin should avoid quinine containing products.


Interactions with Food and other stuff:



Some vegetables as mentioned before are high in vitamin K. eating large quantities or sudden changes in consumption may affect warfarin therapy.






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