Drug Interactions:
Warfarin
Trade Names
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Coumadin
Warfarin (Coumadin) is an anticoagulant used to thin the blood and prevent it from clotting. It is a somewhat dangerous drug that can be affected by many substances, including foods. If you are taking warfarin, we don't recommend taking any herb or supplement except on a physician's advice.
Similar blood-thinning drugs are anisindione (Miradon) and dicumarol.
Alfalfa
Possible Harmful Interaction
The herb alfalfa (Medicago sativa) is promoted for a variety of conditions. The high vitamin K content in alfalfa could, in theory, reduce the effectiveness of warfarin. Vitamin K directly counteracts warfarin's blood-thinning effects.
As a precaution, avoid alfalfa supplements during warfarin therapy except under medical supervision.
Asian Ginseng
Possible Harmful Interaction
The herb Asian ginseng (Panax ginseng) is promoted as an adaptogen, a treatment that helps the body adapt to stress and resist illness in general.
According to one case report, Asian ginseng might decrease warfarin's blood-thinning effects.1
For this reason, if you are taking warfarin, you should not use ginseng without consulting a physician.
Coenzyme Q10 (CoQ10)
Possible Harmful Interaction
Coenzyme Q10 is a vitamin-like substance that plays a fundamental role in the body's energy production.2,3
Coenzyme Q10 (CoQ10) is somewhat similar in structure to vitamin K, and reportedly, it too can reduce the therapeutic effects of warfarin.4 In three case reports, CoQ10 was found to interfere with warfarin's blood-thinning effects.5 A double-blind study found no interaction between CoQ10 and warfarin.59 However, in view of warfarin’s low margin of safety, prudence indicates physician supervision before combining CoQ10 with warfarin.
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.
Danshen
Harmful Interaction
The herb danshen, the root of Salvia miltorrhiza, is used for treating heart disease in traditional Chinese medicine.
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 studies19,20 and a 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 officianale) 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.
Ginkgo appears to reduce the ability of platelets (blood-clotting cells) to stick together.28 Several case reports suggest that this blood-thinning effect of ginkgo may be associated with an increased risk of serious abnormal bleeding episodes in individuals taking the herb.29,30,31 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 A double-blind study found no interaction between ginkgo and warfarin.59 However, in view of warfarin’s low margin of safety, prudence indicates physician supervision before combining ginkgo with warfarin.
Green Tea
Possible Harmful Interaction
Green tea contains strong antioxidant substances and may have cancer-preventive effects.
Because green tea (Camellia sinensis) contains vitamin K, which directly interferes with warfarin's blood-thinning action, drinking large amounts of it might reduce the therapeutic effects of the drug.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 (Coumadin) 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.
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 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 (international units) 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 medical 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
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, OPCs (oligomeric proanthocyanidins), and phosphatidylserine.
View References
Last reviewed August 2002 by Medical Review Board
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