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Soy Protein (generic name)

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Interactions with Drugs

Soy contains "phytoestrogens" (plant-based compounds with weak estrogen-like properties), such as isoflavones. It is not clear if isoflavones stimulate or block the effects of estrogen or both (acting as a "receptor agonist/antagonist"). It is not known if taking soy or soy isoflavone supplements increases or decreases the effects of estrogen on the body, such as the risk of blood clots. It is unclear if taking soy alters the effectiveness of birth control pills containing estrogen.

It is not known what the effects of soy phytoestrogens are on the anti-tumor effects of selective estrogen receptor modulators (SERMs) such as tamoxifen. The effects of aromatase inhibitors such as anastrozole (Arimidex®), exemestane (Aromasin®), or letrozole (Femara®) may be reduced. Because of the potential estrogen-like properties of soy, people receiving these drugs should speak with their oncologists before taking soy in amounts greater than normally found in the diet.

Soy protein may interact with warfarin (Coumadin®), although this potential interaction is not well characterized. Patients taking warfarin should check with a doctor and pharmacist before taking soy supplementation.

Interactions with Herbs and Dietary Supplements

The effects of soy protein or flour on iron absorption are not clear. Studies in the 1980s reported decreases in iron absorption, although more recent research has noted no effects or increased iron absorption in people taking soy. People using iron supplements as well as soy products should consult their qualified healthcare practitioners to follow blood iron levels. Calcium and phosphate levels may be altered.

Some experts believe that there may be a potential interaction between soy extract and Panax ginseng, although this possible interaction is not well understood.

Prebiotics (complex sugars) do not appear to affect how the body absorbs soy. It is unclear if probiotics (commonly found in cultured milk products like yogurt) affect the absorption of soy.


This information is based on a professional level monograph edited and peer-reviewed by contributors to the Natural Standard Research Collaboration ( Ethan Basch, MD, MSc, MPhil (Memorial Sloan-Kettering Cancer Center); Dawn Costa, BA, BS (Natural Standard Research Collaboration); Cynthia Dacey, PharmD (Northeastern University); Paul Hammerness, MD (Massachusetts General Hospital); Shaina Tanguay-Colucci, BS (Natural Standard Research Collaboration); Catherine Ulbricht, PharmD (Massachusetts General Hospital); Christine Ulbricht, BS (University of Massachusetts); Mamta Vora, PharmD (Northeastern University); Wendy Weissner, BA (Natural Standard Research Collaboration); Jen Woods, BS (Natural Standard Research Collaboration).


DISCLAIMER: Natural Standard developed the above evidence-based information based on a thorough systematic review of the available scientific articles. For comprehensive information about alternative and complementary therapies on the professional level, go to Selected references are listed below.

Allison DB, Gadbury G, Schwartz LG, et al. A novel soy-based meal replacement formula for weight loss among obese individuals: a randomized controlled clinical trial. Eur J Clin Nutr 2003;57(4):514-522.

Anderson JW, Luan J, Hoie LH. Structured weight-loss programs: meta-analysis of weight loss at 24 weeks and assessment of effects of intervention intensity. Adv Ther 2004;21(2):61-75.

Campbell CG, Brown BD, Dufner D, et al. Effects of soy or milk protein during a high-fat feeding challenge on oxidative stress, inflammation, and lipids in healthy men. Lipids 2006 Mar;41(3):257-65.

Cohen LA, Crespin JS, Wolper C, et al. Soy isoflavone intake and estrogen excretion patterns in young women: effect of probiotic administration. In Vivo 2007 May-Jun;21(3):507-12.

Fournier LR, Ryan Borchers TA, Robison LM, et al. The effects of soy milk and isoflavone supplements on cognitive performance in healthy, postmenopausal women. J Nutr Health Aging. 2007 Mar-Apr;11(2):155-64.

Giampietro PG, Bruno G, Furcolo G, et al. Soy protein formulas in children: no hormonal effects in long-term feeding. J Pediatr Endocrinol Metab 2004;17(2):191-196.

Izumi T, Saito M, Obata A, et al. Oral intake of soy isoflavone aglycone improves the aged skin of adult women. J Nutr Sci Vitaminol (Tokyo) 2007 Feb;53(1):57-62.

Kerstetter JE, Wall DE, O'Brien KO, et al. Meat and soy protein affect calcium homeostasis in healthy women. J Nutr 2006 Jul;136(7):1890-5.

Koo WW, Hammami M, Margeson DP, et al. Reduced bone mineralization in infants fed palm olein-containing formula: a randomized, double-blinded, prospective trial. Pediatrics 2003;111(5 Pt 1):1017-1023.

Kreijkamp-Kaspers S, Kok L, Grobbee DE, et al. Effect of soy protein containing isoflavones on cognitive function, bone mineral density, and plasma lipids in postmenopausal women: a randomized controlled trial. JAMA 2004;292(1):65-74.

MacGregor CA, Canney PA, Patterson G, et al. A randomised double-blind controlled trial of oral soy supplements versus placebo for treatment of menopausal symptoms in patients with early breast cancer. Eur J Cancer 2005;41(5):708-714.

Nelson HD, Vesco KK, Haney E, et al. Nonhormonal therapies for menopausal hot flashes: systematic review and meta-analysis. JAMA 2006 May 3;295(17):2057-71.

Qin LQ, Xu JY, Wang PY, et al. Soyfood intake in the prevention of breast cancer risk in women: a meta-analysis of observational epidemiological studies. J Nutr Sci Vitaminol (Tokyo) 2006 Dec;52(6):428-36.

Teas J, Braverman LE, Kurzer MS, et al. Seaweed and soy: companion foods in Asian cuisine and their effects on thyroid function in American women. J Med Food 2007 Mar;10(1):90-100.

Tice JA, Ettinger B, Ensrud K, et al. Phytoestrogen supplements for the treatment of hot flashes: the Isoflavone Clover Extract (ICE) Study: a randomized controlled trial. JAMA 2003;290(2):207-214.

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use this medication only for the indication prescribed.

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