In a study of healthy human volunteers, supplementing with American ginseng reduced warfarin's anticoagulant effect, apparently by stimulating the body to accelerate the metabolism of warfarin. People taking warfarin should not take American ginseng, unless supervised by a doctor.

Asian ginseng was associated with a decrease in warfarin activity in a case report. However, in a clinical trial, no interaction was seen between Asian ginseng and warfarin. An animal study also found no significant interaction between warfarin and pure ginseng extract. Nevertheless, persons taking warfarin should consult with a physician knowledgeable about botanical medicines if they are considering taking Asian ginseng or eleuthero/Siberian ginseng (Eleutherococcus senticosus). A 1999 animal study did not reveal any significant interaction between warfarin and pure ginseng extract.

There have been at least five case reports suggesting that cranberry juice increases the activity of warfarin, possibly by inhibiting the breakdown of warfarin in the body. Because of this potential interaction, people taking warfarin should avoid, or limit the intake of, cranberry juice. The U.K. Medicines Authority has advised people taking warfarin to avoid cranberry juice.

Dan shen (Salvia miltiorrhiza), a Chinese herb, was associated with increased warfarin activity in several cases. Dan shen should only be used under close medical supervision by people taking warfarin. Sage (Salvia officinalis), a plant relative of dan shen found in the West, is not associated with interactions involving warfarin.

Devil’s claw (Harpagophytum procumbens) was associated with purpura (bleeding under the skin) in a patient treated with warfarin. However, key details in this case—including other medications taken and the amounts and duration of warfarin and devil’s claw taken—were not reported, making it impossible to evaluate this reported interaction. Until more is known, people taking warfarin should avoid taking devil’s claw.

A 46-year-old woman taking warfarin experienced increased strength of the anticoagulant properties of the drug after starting to use dong quai (Angelica sinensis) for menopause. The daily amount of dong quai was 1,130–2,260 mg per day. Her bleeding tendency returned to normal after discontinuing the dong quai. While little is known about the potential interaction of dong quai and warfarin, women should discuss the use of the herb with a healthcare professional if they are taking an anticoagulant drug and wish to use dong quai.

Asian ginseng was associated with a decrease in warfarin activity in a case report. However, in a clinical trial, no interaction was seen between Asian ginseng and warfarin. An animal study also found no significant interaction between warfarin and pure ginseng extract. Nevertheless, persons taking warfarin should consult with a physician knowledgeable about botanical medicines if they are considering taking Asian ginseng or eleuthero/Siberian ginseng (Eleutherococcus senticosus). A 1999 animal study did not reveal any significant interaction between warfarin and pure ginseng extract.

Although there are no documented cases of feverfew (Tanacetum parthenium) interacting with warfarin in humans, feverfew has been shown to interfere with certain aspects of blood clotting in test tube studies.

Garlic (Allium sativum) has been shown to help prevent atherosclerosis (hardening of the arteries), perhaps by reducing the ability of platelets to stick together. This can result in an increase in the tendency toward bleeding. Standardized extracts have, on rare occasions, been associated with bleeding in people. Garlic extracts have also been associated with two human cases of increased warfarin activity. The extracts were not definitively shown to be the cause of the problem. People taking warfarin should consult with a doctor before taking products containing standardized extracts of garlic or eating more than one clove of garlic daily.

Ginger has been shown to reduce platelet stickiness in test tubes. Although there are no reports of interactions with anticoagulant drugs, people should consult a healthcare professional if they are taking an anticoagulant and wish to use ginger.

Ginkgo extracts may reduce the ability of platelets to stick together, possibly increasing the tendency toward bleeding. Standardized extracts of ginkgo have been associated with two cases of spontaneous bleeding, although the ginkgo extracts were not definitively shown to be the cause of the problem. There are two case reports of people taking warfarin in whom bleeding occurred after the addition of ginkgo. People taking warfarin should consult with a physician knowledgeable about botanical medicines if they are considering taking ginkgo.

There is one reported case of increased bleeding tendency in a woman taking warfarin who also drank 3 to 4 glasses per day of goji berry tea.

In case reports, ingestion of grapefruit seed extract interfered with the effect of warfarin. This inhibitory effect appeared to be due to benzethonium chloride, a synthetic preservative that is added to most grapefruit seed extract products.

Although there are no specific studies demonstrating interactions with anticoagulants, the following herbs contain coumarin-like substances that may interact with warfarin and may cause bleeding. These herbs include angelica root, arnica flower, anise, asafoetida, celery, chamomile, corn silk, fenugreek, horse chestnut, licorice root, lovage root, parsley, passion flower herb, quassia, red clover, rue, sweet clover, and sweet woodruff. Dong quai contains at least six coumarin derivatives, which may account for the interaction noted above. People should consult a healthcare professional if they are taking an anticoagulant and wish to use one of these herbs.

There is one case report in which ingestion of a Chinese herbal tea made from Lycium barbarum appeared to interfere with the effect of warfarin.

Papain, an enzyme extract of papaya, was associated with increased warfarin activity in one patient. Persons taking warfarin should avoid papain supplements until further information about this potential interaction becomes available.

There is one published case report in which the Chinese herbal product quilinggao increased the action of warfarin and apparently contributed to a bleeding episode. There are many different brands of quilinggao, and the composition varies between manufacturers. Individuals taking warfarin should not take quilinggao.

In 1975, a single letter to the Journal of the American Medical Association suggested that vitamin D increases the activity of anticoagulants and that this interaction could prove dangerous. However, there have been no other reports of such an interaction, even though tens of millions of people are taking multivitamins that contain vitamin D. Most doctors typically do not tell patients taking anticoagulant medications to avoid vitamin D.

Coenzyme Q10 (CoQ10) is structurally similar to vitamin K and may affect blood coagulation. Four case reports describe possible interference by CoQ10 with warfarin activity. It remains unknown how common or rare this interaction is. Those taking warfarin should only take CoQ10 with the guidance of their doctor.

One man taking warfarin and one-half to one gallon of green tea (Camellia sinensis) per day developed signs based on laboratory testing suggesting his blood was too thick because the green tea was blocking the effect of warfarin. Removal of the green tea caused normalization of his blood tests. Those taking green tea and warfarin together should have their blood monitored regularly to avert any problems and should consult with a doctor, healthcare practitioner and/or pharmacist before taking any medication.

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

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

According to a preliminary report, volunteers taking 900 mg per day of St. John’s wort were given a single dose of an anticoagulant similar in action to warfarin. There was a significant drop in the amount of the drug measured in the blood. Seven case studies reported to the Medical Products Agency in Sweden also found a decrease in the anticoagulant activity of warfarin when St. John’s wort was taken at the same time. This may have occurred because certain chemicals found in St. John’s wort activate liver enzymes that are involved in the elimination of some drugs. People taking warfarin should consult with their doctor before taking St. John’s wort.

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