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Janet Gray, Ph.D.
Janet Gray, Ph.D.

As author of our 2008 and 2010 State of the Evidence reports, Dr. Gray drives the science behind all our work.

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Bioidentical Hormones

CATEGORY*: Endocrine disruptor

FOUND IN: Medications

THE GIST: Bioidentical hormones match the chemical and molecular structures of hormones produced in the human body, and they are often derived from plant sources. After hormone replacement therapy fell out of favor in 2002, pharmaceutical companies began marketing bioidentical hormones as safe alternatives for alleviating the symptoms of menopause. Unfortunately, few studies have examined the relationship between bioidentical hormones and later development breast cancer, and the treatment may be even riskier than standard HRT, according to the FDA.

State of the Evidence on Bioidentical Hormones

Since publication of the results of major studies indicating that hormone replacement therapy (HRT)  is causally related to post-menopausal breast cancer, many women have turned to alternative sources of hormone therapy to treat their menopausal symptoms in hopes of finding safer options. For many women, this has meant using “bioidentical hormones” of some sort, hoping these substances will mimic the effects of natural hormones without entailing the negative health outcomes associated with traditional HRT.


You can treat many of the symptoms of menopause with exercise and a healthy diet.

More tips for making informed healthcare choices >

Unfortunately there have been very few studies examining the relationship between taking bioidentical hormones and later development of breast cancer. Perhaps more important, and confusing the conversation on this topic, the term “bioidentical hormones” is used in many different ways with potentially different implications for associations with health outcomes (Files, 2011). The most conservative definition, adopted by the Endocrine Society, is “compounds that have exactly the same chemical and molecular structure as hormones that are produced in the human body” (Endocrine Society, 2006). These hormones may be synthesized or derived from plant sources.

A few types of bioidentical hormone composites or their individual components have been tested and approved by the FDA. But the increasingly common use of individually compounded bioidentical hormone regimens has not been tested for safety or associated health outcomes. There is no consistency in the prescribing and providing of individualized compounded formulae, and these practices vary enormously (Files, 2011; Rosenthal, 2008)

The strongest evidence for a lack of association between use of bioidentical hormones and possible development of breast cancer comes from data examining the use of the natural hormone progesterone, instead of synthetic progestins, as part of the HRT regimen (L’Hermite, 2008). Research indicates that increased exposure to natural progesterone does not increase risk for breast cancer and, in some circumstances, might even be protective (Campagnoli, 2005; Holtorf, 2009). In the single large-scale study examining risks for breast cancer in women taking hormone-replacement regimens with either natural progesterone or synthetic progestins compounded with estrogens, use of a progesterone-based replacement was associated with no added risk for breast cancer compared with controls, while women who took combined HRT that included synthetic progestins had a statistically significant increase in risk for developing the disease (Fournier, 2008a). This difference was particularly prevalent in the incidence of estrogen-receptor-positive (ER+) tumors, especially masses that were estrogen-receptor-positive (ER+) and progesterone-receptor-negative (PR-) (Fournier, 2008b).

Less positive news comes from a study comparing the effects of conjugated equine estrogens, the major estrogenic component in traditional combined estrogen-progestin HRT, with the effects of natural estradiol in a primate model (macaques) of post-menopausal breast cancer. In this study, natural estradiol induced greater proliferation of breast epithelial cells than did the conjugated form (Wood, 2008). 

At this point, all authors agree that there is not enough evidence to support or to argue against the health safety of bioidentical hormones, in their many forms, especially with regard to breast cancer risk.

*For chemicals that have been shown to be carcinogens, we provide classifications from two authoritative bodies: the International Agency for Research on Cancer (IARC, an international body) and the National Toxicology Program (NTP, a division of the U.S. Department of Health and Human Services). We have categorized endocrine-disrupting compounds where the body of peer-reviewed research indicates a strong foundation for doing so.