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Herbal Interactions with Warfarin
Mario de Lemos, Pharm. D. student, Rubina Sunderji, Pharm.D.

May 1999 Drug & Therapeutics Newsletter
(C) 1999, CSU-Pharmaceutical Sciences
Vancouver Hospital & Health Sciences Centre

Several case reports of interactions between various herbal products and warfarin have been reported recently.1-12 As warfarin has a narrow therapeutic window, it is important to be aware of these potential interactions to prevent anticoagulant under- or overdosage. Table 3 summarizes interactions between commonly used herbal products and warfarin.1-17

Case reports

Heavy use of garlic (Allium sativum) supplement was associated with one case of spontaneous spinal epidural hematoma1 and two cases of prolonged clotting time with increased post-operative bleeding.2,3 The authors suggest that garlic supplements should be stopped for at least 10-14 days prior to surgery.2,3 Although none of these patients were receiving concurrent warfarin, it is prudent to avoid garlic supplements in patients taking warfarin. Ginkgo (Ginkgo biloba) at a daily dose of 80-160 mg was associated with 5 cases of hemorrhage: subdural hematoma (2)4,5, hyphema (1)6, intracerebral hemorrhage (1)7 and subarachnoid hemorrhage (1)8. In three of these cases, the time to onset of the adverse effect was relatively short from the time of starting Ginkgo, from 1 week5 to less than 2 months6,7. Although only one case involved concurrent warfarin7 and another involved aspirin6, patients taking warfarin should avoid Ginkgo due to the possible increased risk of hemorrhage. Danshen (Salvia miltiorrhiza) was found to potentiate anticoagulation in three patients stabilized on warfarin.9-11 The INR was increased from around 2.0-3.0 to over 5.5 after taking danshen for 2-4 weeks. Ginseng was reported to reduce INR from 3.0-4.0 to 1.5 in a patient stabilized on warfarin after taking ginseng capsules for 2 weeks.12

Other commonly used herbal products which have NOT been reported to interfere with warfarin include alfalfa, bee pollen, betel nut, blue-green algae, chamomile tea, devil’s claw, dong quai, evening primrose oil, kelp, milk thisle, royal jelly, saw palmetto, St. John’s wort, and valerian.13-16 A lack of reported cases however does not imply the safety of these products when administered with concurrent warfarin, as few of these combinations have been studied specifically to determine the impact of herbal medications on anticoagulation. For example, some herbs (e.g. chamomile14) contain coumarins although their effects on the anticoagulant system is unknown.17

Table 3. Drug Interactions between Warfarin and Herbal Products


Common Usage


Evidence of Interaction


Coronary heart disease, supplement post surgery

8 INR by 9 warfarin elimination; also 9 platelet aggregation

case reports (see text)

Echinacea purpurea19

Stimulates immune response to infections

8 INR by

9 warfarin metabolism by 9 CYP3A4 liver enzyme



Migraine, rheumatoid arthritis

8 risk of bleed by 9 platelet aggregation



Atherosclerosis, hypertension

8 risk of bleed by 9 platelet aggregation

case reports (see text)



8 risk of bleed by 9 platelet aggregation


Gingko biloba4-8


8 risk of bleed by 9 platelet aggregation

case reports (see text)


Stress, physical endurance

9 INR, unknown mechanism

case reports

(see text)

Green tea & herbal teas made with tonka beans, melilot or woodruff20


9 INR due to high vitamin K content


† Interaction based on in vitro studies with herb


Due to the limited data available, patients should be advised about the potential interaction of herbal products with warfarin.


  1. Rose KD et al. Spontaneous spinal epidural hematoma with associated platelet dysfunction from excessive garlic ingestion: a case report. Neurosurgery 1990;26:880-2.

  2. Burnham BE. Garlic as a possible risk for postoperative bleeding. Plast Reconstr Surg 1995;95:211.

  3. German K et al. Garlic and the risk of TURP bleeding. Br J Urol 1995;76:518.

  4. Rowin J et al. Spontaneous bilateral subdural hematomas associated with chronic Ginkgo biloba ingestion. Neurology 1996;46:1775-6.

  5. Gilbert GJ. Ginkgo biloba. Neurology 1997;48:1137.

  6. Rosenblatt M, Mindel J. Spontaneous hyphema associated with ingestion of Ginkgo biloba extract. N Engl J Med 1997;336:1108.

  7. Matthews MK. Association of Ginkgo biloba with intracerebral hemorrhage. Neurology 1998;50:1933-4.

  8. Vale S. Subarachnoid haemorrhage associated with Ginkgo biloba. Lancet 1998;325:36.

  9. Tam LS et al. Warfarin interactions with Chinese traditional medicines: danshen and methyl salicylate medicated oil. Aust NZ J Med 1995;25:258.

  10. Yu CM et al. Chinese herbs and warfarin potentiation by ‘Danshen’. J Intern Med 1997;241:337-0.

  11. Izzat MB et al. A taste of chinese medicine! Ann Thor Surg 1998;66:941-2.

  12. Janetzky K et al. Probable interaction between warfarin and ginseng. Am J Health-Syst Pharm 1997;54:629-3.

  13. Cadario B. The bottom line on herbal products. BC Pharmacy June 1998.

  14. Miller LG. Herbal medicinals. Selected clinical considerations focusing on known or potential drug-herb interactions. Arch Intern Med 1998;158:2220-11.

  15. Briggs C. Alfalfa. Can Pharm J 1994;127:84-85, 115.

  16. Burnham BT (ed). The review of natural products. St. Louis: Facts and Comparisons;1992. Betel Nut, Dong quai.
  17. Hoult JRS et al. Pharmacological and biochemical actions of simple coumarins: natural products with therapeutic potential. Gen Pharmacol 1996;27:713-22.
  18. Chan K et al. The effects of Danshen (Salvia miltiorrhiza) on warfarin pharmacodynamics and pharmacokinetic of warfarin enantiomers in rats. J Pharm Pharmacol 1995;47:402-6.
  19. Winslow LC et al. Herbs as medicines. Arch Intern Med 1998;158:2192-9.
  20. Verduyn I. Diet guidelines while taking Coumadin or warfarin. Vancouver (BC): Dept. of Food & Nutrition Services, Vancouver General Hospital; May 1997. Patient Information Pamphlet #PE-5600.

Further references available on request.