Herbal interaction with warfarin.

To the Editor: The recent article on preventing warfarin-related bleeding (1) is an excellent review on the subject. Unfortunately, the author failed to discuss herbal medicines, many of which interact with warfarin. (2-5) Because 33% of American patients take herbal medicines (2) and as warfarin is one of the most commonly used cardiovascular drugs, physicians need to be aware of herbal interactions with warfarin.

Several herbs may interact with warfarin, either increasing or decreasing the anticoagulant effect. (2-5) For example, danshen, an herbal drug commonly used for various symptoms of coronary heart disease including angina, myocardial infarction, and heart failure, (2) and dong quai, commonly used for menopausal symptoms, (3) both potentiate the anticoagulant effect of warfarin. Ginseng, however, actually decreases the anticoagulant effect of warfarin. (4) To further complicate the situation, danshen is taken in a variety of forms: by inhalation and by incorporation into some Chinese cigarettes. (3) Therefore, in taking a history, the physician should not only ask which herbs the patient is taking but also what brand of cigarettes he or she is smoking. (3)

The most recent addition to the list of herb-warfarin interaction is soy milk, which may cause subtherapeutic international normalized ratio values. (5) Although soy milk is not actually an herb, it is widely advocated as a nutritional supplement by practitioners of both Western and traditional medicine. Soy milk is also used as a health food, not only for its antihypertensive and antihyperlipidemic properties, but also as a natural alternative to hormone therapy. (5) This use has recently attracted attention in light of the disquieting findings from the Women's Health Initiative trial, which demonstrated an increasing incidence of cardiovascular events and breast cancer in women taking hormonal therapy. (5)

The use of complementary or alternative medicine in the United States has become increasingly popular. Therefore, every physician should be aware of possible drug-herb interactions especially in patients on warfarin who experience excessive bleeding or unexpected prolongation of international normalized ratio and who have previously been under good anticoagulant control. (4,5)

Tsung O. Cheng MD

George Washington University 

Medical Center

Washington, DC

References

1. Glasheen JJ. Preventing warfarin-related bleeding. South Med J 2005;98:96-103.

2. Cheng TO. Herbal interactions with cardiac drugs. Arch Intern Med 2000;160:870-871.

3. Cheng TO. Ask patients which herbal drugs they smoke as well as eat. West J Med 2000;172:82.

4. Cheng TO. Ginseng-warfarin interaction. Am Coll Cardiol Curr Journal Review 2000;9:84.

5. Cheng TO. Potential interaction between soy milk and warfarin. Am Fam Phys 2004;70:1231. Letters to the Editor

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