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Are there any interactions with phytoestrogens?

Phytoestrogens, which are plant-based substances chemically similar to the human hormone estrogen, are available in several supplements. The most common include black cohosh, dong quai, and soy.

Women taking birth control pills, estrogen therapy, azathioprine (Imuran), cyclosporine (Sandimmune, Neoral), or blood pressure medicines should not take black cohosh.

Dong quai may interact with anticoagulants (blood thinners) like warfarin (brand name Coumadin) and daily aspirin therapy.

Soy may prevent the thyroid medicine levothyroxine (Synthroid) from being absorbed into the body. If you are taking levothyroxine and soy, it's best to separate your levothyroxine dose from any soy supplements by at least two hours. You should talk to your doctor if you are taking iron supplements or warfarin (Coumadin).

Concentrated phytoestrogen supplements may interfere with the use of tamoxifen (Nolvadex), so please make sure you tell you doctor if you are taking this medication.

Sources:
Drug Facts and Comparisons. Facts & Comparisons, 2007.
Micromedex® Healthcare Series. Thomson Micromedex, 2007.
Peirce, A. The American Pharmaceutical Association Practical Guide to Natural Medicines. William Morrow & Co., 1999.
Kuhn, M.A. Herbs and Menopause. Medical Educational Services, Inc., 1997.
Kuhn, M.A. Herbs, Drugs & the Body. Medical Educational Services, Inc., 1999.
Natural Prescriptions for Women. Rodale Press, Inc., 1998.
Synthroid product information, Knoll Pharmaceutical Company, 2000.
Murkies AL. “What is the role of phytoestrogens in treating menopausal symptoms?” Medical Journal of Australia 173 (2000).

This answer prepared 3/8/2001.
This information updated 1/22/2007.