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Generally speaking, phytoestrogens in plant-based foods are safe to consume if used as part of a balanced diet. By contrast, little is known about the long-term safety of phytoestrogen supplements.
Soy isoflavones, the type most commonly used in phytoestrogen supplements, may cause stomach upset, bloating, gas, and nausea. Allergic reactions are rare but may incur in people with a known soy allergy.
Due to their estrogen-like effect, phytoestrogen supplements should be avoided in women with estrogen-receptor-positive breast cancer or people with other hormone-sensitive cancers, including endometrial, ovarian, and prostate cancer.
Phytoestrogen supplements should not be used with tamoxifen. By binding to the same estrogen receptors used by tamoxifen, phytoestrogens can "compete" with the drug and reduce its efficacy. Doing so may increase the risk of breast cancer recurrence.
The safety of phytoestrogen supplements in pregnancy is unknown. To be safe, avoid taking any supplemental form of phytoestrogen if you are pregnant or breastfeeding. *
