Genistein / Soy

Also known by these names

Soy foods may have several other labels:

  • Bean curd
  • Edamame
  • Kinoko flour
  • Kyodofu
  • Miso
  • Natto
  • Okara
  • Shoyu sauce
  • Soy milk
  • Soya
  • Supro
  • Tamari
  • Tempeh
  • Textured vegetable protein
  • Tofu
  • Yakidofu
  • Yuba

Key Points

  • Before using this therapy, consult your oncology team about interactions with other treatments and therapies. Also make sure this therapy is safe for use with any other medical conditions you may have.
  • Soy contains large amounts of isoflavones, including genistein.
  • In laboratory studies, isoflavones have slowed the growth of several types of cancer.
  • BCCT is interested in soy because it may reduce risk of breast, prostate and lung cancers, plus colorectal cancer in women. It may also prolong survival and reduce recurrence in some cancer patients.
  • Soy can increase risk of bladder cancer.
  • Cell and animal studies have shown that adding soy food nutrients to tamoxifen inhibits the growth of estrogen receptor positive (ER+) breast cancer cells.
  • Not enough information about soy supplements, such as concentrated isoflavones, has been assessed to determine if they are safe.
  • Soy or genistein can interfere or interact with a few other prescription medications.


Laura Pole, RN, MSN, OCNS, BCCT Senior Researcher

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Nancy Hepp, MS, BCCT Project Manager

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Last updated April 29, 2021.

The beans of the soy plant contain isoflavones, a type of phytoestrogens which are chemical structures similar to the estrogen made in our bodies. However, phytoestrogens are not the same as female estrogens. Soy foods do not contain estrogen,1 and is not likely to have estrogen-like effects on hormonally responsive tissues.2

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Treating the Cancer

Working against cancer growth or spread, improving survival, or working with other treatments or therapies to improve their anticancer action

Clinical Evidence

Breast Cancer

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Lung Cancer

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Ovarian Cancer

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Prostate Cancer

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Lab and Animal Evidence

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Managing Side Effects and Promoting Wellness

Managing or relieving side effects or symptoms, reducing treatment toxicity, supporting quality of life or promoting general well-being

Soy isoflavones protected normal tissues and organs against radiation damage in prostate cancer patients23 and reduced urinary, bowel, and sexual adverse symptoms induced by radiation therapy for prostate cancer in a small pilot study.24

Soy isoflavones show slight and slow effects in attenuating menopausal hot flashes compared with estradiol25 but greater effects than placebo in some studies.26 but not all.27 The 2015 update of the North American Menopause Society's evidence-based position on nonhormonal management of menopause-associated vasomotor symptoms found that S-equol derivatives of soy isoflavones may be beneficial for alleviating hot flashes.Nonhormonal management of menopause-associated vasomotor symptoms: 2015 position statement of The North American Menopause Society. Menopause. 2015;22:1155–1172.

Soy products have protected mucosal cells against methotrexate toxicity in animals, potentially reducing mucositis, stomatitis, diarrhea, decreased nutrient absorption, translocation of gastrointestinal bacteria, and anorexia.28 Consistent with this, a pilot study in children showed less myelosuppression, mucositis, and infection when genistein was taken with chemotherapy, and patients who received abdominal radiation reported less pain and diarrhea when they took the genistein supplement.29

Reducing Risk

Reducing the risk of developing cancer or the risk of recurrence

Clinical Evidence

Soy consumption is associated with reduced risk of some cancers but increased risk of others.

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Lab and Animal Evidence

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Optimizing Your Terrain

Creating an environment within your body that does not support cancer development, growth or spread

Clinical Evidence

Evidence is generally favorable but sometimes inconclusive. Soy consumption is associated with reduced inflammation40 and improved hormone balance,41 although the specific interactions with hormones are still under investigation. 

Genestein shows anti-inflammatory and antioxidant effects42


Soybeans and foods derived from soybeans are widely available. Extracts are sold as dietary supplements.


While in the past patients on tamoxifen were advised to avoid soy and genistein, more recent data suggest that soy does not reduce effects of tamoxifen and may even protect against estrogen-related endometrial cancers.

Soy Foods

Soy foods are generally well tolerated by patient. “Regular ingestion of moderate amounts of soy daily seems to be safe—especially with the traditional fermented soy foods such as miso, tempeh, and natto when made from organic non-GMO soybeans."43

High intake of soy isoflavones or soy-based food during infancy and in adulthood is associated with an increased risk of uterine fibroids in premenopausal women.44

In HER2 positive breast cancer patients, soy consumption has been associated with a greater risk of recurrence,45 and overexpression of genes that promote cell proliferation.46 Naturopathic oncologist and BCCT advisor Lise Alschuler, ND, FABNO, recommends limiting consumption of soy products to one serving daily for women with a history or high risk of estrogen-dependent breast cancer.

A diet high in soy, when combined with iodine deficiency, can increase the risk of thyroid cancer. Therefore, a diet high in soy products may be contraindicated for those diagnosed with thyroid cancer or at high risk of thyroid cancer.47 Soy consumption may be associated with increased risks of bladder cancer, although evidence is contradictory across studies.48

Soy Supplements

Not enough information about soy supplements, such as concentrated isoflavones, has been assessed to determine if they are safe.49 Because phytoestrogens and their active metabolites can remain in food/meat, they may influence the hormonal balance of those consuming the food. Phytoestrogens may affect fertility, sexual development and behavior.50

A 2016 review found that estrogenic isoflavone-based food supplements are associate with both increased risk and decreased risk of breast cancer. The authors concluded that benefits do not outweigh the risks to postmenopausal women.51 This is consistent with animal studies showing increased metastasis with the use of soy supplements.52

Consumption of soy supplements may cause mild stomach and intestinal side effects such as constipation, bloating and nausea and may also cause allergic reactions involving rash, itching, and anaphylaxis in some people.53

Interactions with a few prescription medications are noted. These include tamoxifen and aromatase inhibitors.54

Soy supplementation had been proposed as a cause of endometrial hyperplasia, but several studies have found no increased risk.55 Animal evidence (see above) shows a protective effect against endometrial cancer.56

Further adverse effects and side effects are noted on the Memorial Sloan Kettering About Herbs webpage: Soy. However, the About Herbs site does not reflect recent results regarding finding of effects of soy consumption for women taking anti-estrogen hormone therapy for breast cancer.


BCCT does not recommend therapies or doses, but provides information for patients and providers to consider as part of a complete treatment plan. Patients should discuss therapies with their physicians, as contraindications, interactions and side effects must be evaluated. Levels of active ingredients of natural products can vary widely between and even within products. See Quality and Sources of Herbs, Supplements and Other Natural Products.

A 2018 review offers this guidance: "Women with breast cancer can be advised to enjoy a reasonable amount of whole soy foods, while avoiding supplemental soy protein and isoflavone isolates. A reasonable amount can be deduced from the average soy consumption in Asian populations, which provides 10–20 mg of soy isoflavones per person daily. This is the amount found in 30 g of whole soy products such as tofu."57

Further recommendations are available from these sources:

Integrative Programs, Protocols and Medical Systems

For more information about programs and protocols, see our Integrative Programs and Protocols page.

Non-cancer Uses of Soy/Genistein

BCCT has not reviewed the effectiveness of this therapy for non-cancer uses.

  • Prevent heart disease
  • Reduce high cholesterol
  • Treat menopause symptoms
  • Prevent bone loss

This summary draws from the Memorial Sloan Kettering Cancer Center’s About Herbs and National Cancer Institute’s PDQ® websites, the American Institute for Cancer Research website, and other sources as noted.

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