Alfalfa

Possible Harmful Interaction

The herb alfalfa (Medicago sativa) is promoted for a variety of conditions. The relatively high vitamin K content in alfalfa could reduce the effectiveness of warfarin. Vitamin K directly counteracts warfarin's blood-thinning effects. Since the amount of vitamin K in alfalfa varies widely, it is difficult to give an exact safe upper dose.

As a precaution, avoid alfalfa supplements during warfarin therapy except under medical supervision.

Chamomile

Possible Harmful Interaction

The herb chamomile contains substances in the coumarin family. Some coumarins have blood thinning actions that could interact with warfarin. One case report exists of a person in which it appears that combined use of chamomile and warfarin led to internal bleeding. 74

Chondroitin

Possible Harmful Interaction

Based on chondroitin’s chemical similarity to the anticoagulant drug heparin, it has been suggested that chondroitin might have anticoagulant effects as well. There are no case reports of any problems relating to this, and studies suggest that chondroitin has at most a mild anticoagulant effect. 62 Nonetheless, prudence suggests that chondroitin should not be combined with warfarin except under physician supervision.

Coenzyme Q 10 (CoQ 10 )

Possible Harmful Interaction

CoQ ₁₀ is a vitamin-like substance that plays a fundamental role in the body's energy production. 2,3

This substance is somewhat similar in structure to vitamin K, and reportedly it, too, can reduce the therapeutic effects of warfarin. 4 In three case reports, CoQ ₁₀ was found to interfere with warfarin's blood-thinning effects. 5 A double-blind study found no interaction between CoQ ₁₀ and warfarin. 59 However, in view of warfarin’s low margin of safety, prudence indicates physician supervision before combining CoQ ₁₀ with warfarin.

Cranberry

Possible Harmful interaction

Several case reports suggest that cranberry juice can increase warfarin’s action, causing dangerous and potentially fatal bleeding problems. 65, 82 However, formal studies have failed to find evidence of such an interaction. 80, 81 Nonetheless, prudence would suggest continued caution at this time, especially when taking cranberry juice in dosages higher than 8 oz daily.

Danshen

Possible Harmful Interaction

The herb danshen, the root of Salvia miltiorrhiza, is used in traditional Chinese medicine for treating heart disease.

Preliminary evidence, including several case reports, suggests that danshen can dangerously increase the effects of warfarin and cause significant bleeding problems. 6,7,8

Therefore, if you take warfarin, you should avoid danshen except under a physician's supervision.

Devil's Claw

Possible Harmful Interaction

The herb devil's claw ( Harpogophytum procumbens ) is used for various types of arthritis and digestive problems.

According to one case report, devil's claw might increase the risk of abnormal bleeding when taken with warfarin. 9

As a precaution, you should probably not combine devil's claw and warfarin except under a physician's supervision.

Dong Quai

Possible Harmful Interaction

The herb dong quai ( Angelica sinensis ) is used for menstrual disorders.

According to one case report, dong quai may add to the blood-thinning effects of warfarin, thus increasing the risk of abnormal bleeding. 10

You should probably avoid combining dong quai and warfarin without medical supervision.

Feverfew

Possible Harmful Interaction

The herb feverfew ( Tanacetum parthenium ) is primarily used for the prevention and treatment of migraine headaches.

In vitro studies suggest that feverfew thins the blood by interfering with the ability of blood platelets to clump together. 11-14 This raises the concern that feverfew might increase the risk of abnormal bleeding when combined with warfarin. However, there is as yet no evidence that the blood-thinning effect of feverfew is significant in humans. 15

Though an additive effect of feverfew and warfarin appears to be theoretical at this time, it may be best to avoid this combination except under medical supervision.

Garlic

Possible Harmful Interaction

The herb garlic (Allium sativum) is taken to lower cholesterol, among many other proposed uses.

One of the possible side effects of garlic is an increased tendency to bleed. 16,17 This blood-thinning effect has been demonstrated in a double-blind trial of garlic in 60 volunteers, 18 as well as in other studies 19,20 and one case report. 21

According to two other case reports, the blood-thinning effects of warfarin were greatly enhanced in individuals taking garlic. 22 This could amplify the risk of bleeding problems.

Based on these findings, you should avoid combining garlic and warfarin except under a physician's supervision.

Ginger

Possible Harmful Interaction

The herb ginger ( Zingiber officinale ) is used for nausea associated with motion sickness, morning sickness in pregnancy, and the postsurgical period.

Ginger appears to thin the blood by interfering with the ability of blood platelets to clump together. 23,24 As with feverfew, this raises the concern that ginger might increase the risk of abnormal bleeding when taken with warfarin. However, there is no evidence at present that the blood-thinning effect of ginger is significant in humans. 25,26,27

Though an additive effect of ginger and warfarin appears to be theoretical based on current evidence, it may be best to avoid this combination except under medical supervision. Ginger flavored drinks should not present a problem, but candies containing whole dried ginger are potentially of concern.

Ginkgo

Possible Harmful Interaction

The herb ginkgo ( Ginkgo biloba ) has been used to treat Alzheimer's disease and ordinary age-related memory loss, among many other uses.

Inconsistent evidence suggests that ginkgo might reduce the ability of platelets (blood-clotting cells) to stick together. 28,66,67,68,76 In addition, several case reports suggest that use of ginkgo may be associated with an increased risk of serious abnormal bleeding episodes in individuals taking the herb. 29,30,31,69,70 These findings raise concern that ginkgo might add to the blood-thinning effects of warfarin, and there is one report of abnormal bleeding in an individual who had been taking the herb and drug together. 32 However, two double-blind studies found no interaction between ginkgo and warfarin. 59,77 These findings are reassuring. Nonetheless, in view of warfarin’s low margin of safety, prudence indicates physician supervision before combining ginkgo with warfarin.

Low Carb, High Protein Diet

Possible Harmful Interaction

Low carbohydrate, high-protein diets have been advocated for weight loss. According to two case reports, adoption of such diets may decrease the effectiveness of warfarin, possibly by increasing blood levels of a substance called albumin that might tend to bind and inactivate warfarin in the body. 73

Green Tea

Possible Harmful Interaction in Very High Doses

Dried green tea leaf contains significant levels of vitamin K on a per-weight basis. On this basis, it has been stated that people using blood thinners in the warfarin family should avoid green tea. However, green tea taken as a beverage provides such small amounts of the vitamin that the risk seems minimal for normal consumption. There is one case report of problems that developed in a person on warfarin who consumed as much as a gallon of green tea daily. 33

Ipriflavone

Possible Harmful Interaction

Ipriflavone, a synthetic isoflavone that slows bone breakdown, is used to treat osteoporosis.

Warfarin use increases the risk of osteoporosis. Because ipriflavone has been found to help prevent osteoporosis in certain circumstances, you might be tempted to consider taking this supplement while you use warfarin. However, some evidence indicates that ipriflavone might interfere with the body's normal breakdown of warfarin. 34 This could raise the levels of warfarin in your body and increase the risk of abnormal bleeding.

If you try this combination, you need to do so under physician supervision.

Papain, Bromelain

Possible Harmful Interaction

One case report suggests that papain, a digestive enzyme found in papaya extract ( Carica papaya ), might add to warfarin's blood-thinning effect. 35

Vinpocetine

Possible Harmful Interaction

The substance vinpocetine is sold as a dietary supplement for the treatment of age-related memory loss and impaired mental function. Vinpocetine is thought to inhibit blood platelets from forming clots. 60 For this reason, it should not be combined with medications or natural substances that impair the blood’s ability to clot normally, as this may lead to excessive bleeding. One study found only a minimal interaction between the blood-thinning drug warfarin and vinpocetine (and it actually involved an increased tendency for blood clotting), but prudence dictates caution anyway. 44

PC-SPES

Possible Harmful Interaction

PC-SPES is an herbal combination that has shown promise for the treatment of prostate cancer. One case report suggests that PC-SPES might increase risk of bleeding complications if combined with blood-thinning medications. 56 Subsequent evidence has indicated that PC-SPES actually contains warfarin, making this interaction inevitable. 61

Policosanol

Possible Harmful Interaction

Policosanol, derived from sugarcane, is used to reduce cholesterol levels. It also interferes with platelet clumping, creating a risk of interactions with blood-thinning drugs.

For example, a 30-day, double-blind, placebo-controlled trial of 27 individuals with high cholesterol levels found that policosanol at 10 mg daily markedly reduced the ability of blood platelets to clump together. 37 Another double-blind, placebo-controlled study of 37 healthy volunteers found evidence that the blood-thinning effect of policosanol increased as the dose was increased—the larger the policosanol dose, the greater the effect. 38 Yet another double-blind, placebo-controlled study of 43 healthy volunteers compared the effects of policosanol (20 mg daily), the blood-thinner aspirin (100 mg daily), and policosanol and aspirin combined at these same doses. 39 The results again showed that policosanol substantially reduced the ability of blood platelets to stick together, and that the combined therapy exhibited additive effects.

Based on these findings, you should not combine warfarin and policosanol except under medical supervision.

Reishi

Possible Harmful Interaction

One study suggests that reishi impairs platelet clumping. 58 This creates the potential for an interaction with any blood-thinning medication.

Royal Jelly

Possible Harmful Interaction

One case report indicates that use of royal jelly can increase the effectiveness of warfarin, creating risk of bleeding. 83

Soy

Possible Harmful Interaction

One case report indicates that soy milk might decrease warfarin’s effectiveness. 71

St. John's Wort

Possible Harmful Interaction

The herb St. John's wort ( Hypericum perforatum ) is primarily used to treat mild to moderate depression.

Evidence suggests that St. John's wort may interfere with warfarin, possibly requiring an increased dosage of the drug to maintain the proper therapeutic effect. 40,41,42,72 Seven cases have been reported in which the blood-thinning effects of warfarin have been impaired in individuals taking St. John's wort. 43

A "hidden" risk lies in this type of interaction. Suppose your physician has raised the warfarin dose to take into account the effect of St. John's wort in holding down drug levels. If you then stop taking the herbal product, it would be like releasing the brakes, and your warfarin levels could surge dangerously high.

For these reasons, if you take warfarin, avoid St. John's wort except under a physician's supervision.

Vitamin A

Possible Harmful Interaction

Supplemental vitamin A might increase the blood-thinning effects of warfarin, and this could potentially lead to an increased risk of abnormal bleeding. 45

For this reason, it may be best to avoid combining vitamin A with warfarin unless supervised by a physician.

Vitamin C

Possible Harmful Interaction

Vitamin C taken in high dosages (more than 1,000 mg daily) has been reported to reduce the blood-thinning effect of warfarin. 46-49 In one case, the person was taking 1,000 mg of vitamin C daily; another involved huge megadoses (about 16,000 mg daily).

As a precaution, if you take warfarin, consult with your physician before taking high-dose vitamin C supplements.

Vitamin E

Possible Harmful Interaction

On the basis that vitamin E "thins" the blood, 50 it has been suggested not to combine vitamin E with warfarin. However, a 4-week, double-blind study of 25 individuals taking warfarin found no additive effect. 51 None of the participants taking vitamin E at a daily dose of 800 or 1,200 IU showed an increased risk for abnormal bleeding.

In contrast, a case report indicated that vitamin E (800 IU daily) added to the effects of warfarin and resulted in abnormal bleeding. 52 Because this effect did not become apparent until the fourth week, it is possible that problems might take longer to develop than the 4-week period covered by the double-blind study, or that certain individuals might be more prone to an interaction. An unpublished, 30-day study of three volunteers taking a warfarin-like drug also found an additive effect with only 42 IU of vitamin E daily. 53

Though the evidence supporting a possible interaction is scanty, it is best not to risk serious bleeding problems. Avoid combining vitamin E with warfarin except under the supervision of a physician.

Vitamin K

Possible Harmful Interaction

Vitamin K is an antidote to warfarin—it directly counteracts warfarin's blood-thinning effects. This is true for both supplemental vitamin K and foods high in vitamin K. For this reason, eating more vitamin K-rich vegetables can decrease warfarin's therapeutic effect, and eating less of these foods can increase the drug's effect. 54,55 Either situation can lead to potential life-threatening complications.

Therefore, once you are established on a certain dose of warfarin, you should not change your usual intake of vitamin K without informing your physician.

One study suggests a novel way of using this effect deliberately. 84 Researchers gave people on warfarin a fixed daily dose of vitamin K in order to override the changes in warfarin action caused by the natural variation in day-to-day dietary vitamin K consumption. The results were positive: INR values—the standard measurement of warfarin’s blood thinning effect—became more stable. Needless to say, however, this method should not be used except under close physician supervision.

White Willow

Possible Harmful Interaction

The herb white willow ( Salix alba ), also known as willow bark, is used to treat pain and fever. White willow contains a substance that is converted by the body into a salicylate similar to the blood-thinner aspirin.

Because white willow, like aspirin, may enhance the blood-thinning effects of warfarin, this combination should be avoided unless medically supervised.

Other Herbs and Supplements

Possible Harmful Interaction

One case report suggests that a combination of the herbs boldo and fenugreek increased the effects of warfarin. 57 Another isolated case report suggests that the same can happen when fish oil is combined with warfarin. 78

Based on their known effects or the effects of their constituents, the following herbs and supplements might not be safe to combine with warfarin, though this has not been proven: chamomile ( Matricaria recutita ), Coleus forskohlii , ginger ( Zingiber officinale ), horse chestnut ( Aesculus hippocastanum ), papaya ( Carica papaya ), red clover ( Trifolium pratense ), and reishi ( Ganoderma lucidum ); mesoglycan , fish oil , oligomeric proanthocyanidins (OPCs), and phosphatidylserine .