Does Hormone Replacement Therapy Cause Breast in Women?

This is a controversial subject, mostly because of media hype and some legitimate scientific concern, but actually, it is not that complicated. So what is the real answer? Yes, no, maybe? In this article, you may be surprised by what you will learn, and how it can help you make the best decision for yourself.

The short summary is this: Synthetic versions of hormones, particularly synthetic progesterone, DOES increase the risk for breast cancer. But natural Bioidentical Hormone Replacement Therapy does NOT increase the risk for breast cancer.

In the past decade, a lot has changed with the perceptive safety of hormone replacement therapy and the support of bioidentical hormones. In fact, multiple major medical organizations now endorse hormone therapy as a safe and effective treatment option. A 2015 survey conducted by the North American Menopause Society found that more than 60 percent of doctors support the use of bioidentical hormones. Richard Peto, a professor of statistics and epidemiology at Oxford University has been quoted saying, “Hormone-sensitive cancers are still three times as common in HRT users as in non-users or ex-users.” In other words, he is saying that the risk for breast cancer is the same for post menopausal women, whether they use hormones or not. But, there is more to the story. If we consider women who are only using bioidentical hormones, the risk is actually lower. There are at least seven studies that have shown that bioidentical progesterone inhibits estrogen-stimulated breast cells. 1-7

The take home message here is this: Bioidentical hormones simply DO NOT increase the risk for breast cancer, and MAY actually decrease the risk and fight existing breast cancer.

By the Way, What’s a “Bioidentical Hormone”?

This simply implies that the chemical structure of the hormone medication is an exact biochemical replica of what is found in the human body. They are made from soybeans or the Mexican wild yam (Dioscorea villosa) and are chemically created in the lab. Compounding Pharmacies then use this USP (United States Pharmacopeia) approved compound to create the specific dose that the physician prescribes.

At The Hormone Zone, we prefer to use natural bioidentical hormones for our patients for their sex hormone needs for menopause, male low testosterone, thyroid or adrenal hormones.

The Infamous Women’s Health Initiative

This is the study that really created the controversy. This large study of thousands of women, that started in 1991 was analyzing Permarin, Prempro, and Provera, all synthetic estrogens/progestins. In 2002, the study was abruptly halted because of concern over an increased risk of breast cancer and uterine cancer.

The most important thing to note about this large study was that they were studying synthetic versions of both estrogen and progesterone, not natural bioidentical versions. The newest information about this study has shown that the estrogen did NOT increase the risk for breast cancer. However, it was very clear then, and also clear now, that synthetic progesterones (i.e. medroxyprogesterone) increase the risk for breast cancer.

The Not-So-Famous (At least in the US) French Cohort Study

We do not hear about this study often in the US, but it is a very large study, equal in size and scope to the WHI Study, but they included natural bioidentical hormones in the study. They followed over 80,000 women over 15 years to analyze the risk of both synthetic HRT and bioidentical hormones, and their relationship to breast cancer. Just as was seen in the WHI Study, the increased risk of breast cancer points to the synthetic progesterones. In fact, those women who had been using a bioidentical progesterone (micronized progesterone, like what is typically prescribed at The Hormone Zone) did NOT have an increased risk of breast cancer at all.

To read the French Cohort Study, explore HERE

The Bottom Line

Breast cancer is not a “one thing did it” disease. The cause is multifactorial and is never from hormones alone, synthetic or otherwise. The goal for any patient at The Hormone Zone is to discuss options and lifestyle changes to decrease the overall risk of breast cancer. Ultimately, Bioidentical Hormones are safer alternatives than their synthetic versions. Since they are compounded, the dosing can be specifically altered to produce superior outcomes for patient symptom relief, minimize side effects, and maximize health outcomes. Rest assured that natural Bioidentical Hormones, in the hands of the experts at The Hormone Zone, will be the safe solution for your hormone needs.

For More Information on BHRT and Breast Cancer Risk from another hormone expert, explore a detailed White Paper by Dr. Mark Gordon, available HERE


1. Chang KJ, Lee TY, Linares-Cruz G, Fournier S, de Ligniéres B. Influences of percutaneous administration of estradiol and progesterone on human breast epithelial cell cycle in vivo. Fertil Steril. 1995;63(4):785–791.

2. Foidart JM, Colin C, Denoo X, et al. Estradiol and progesterone regulate the proliferation of human breast epithelial cells. Fertil Steril. 1998;69(5):963–969.

3. Mueck AO, Seeger H, Wallwiener D. Comparison of proliferative effects of estradiol and conjugated equine estrogens on human breast cancer cells and impact of continuous combined progestogen addition. Climacteric. 2003;6(3):221–227. 30

4. Inoh A, Kamiya K, Fujii Y, Yokoro K. Protective effects of progesterone and tamoxifen in estrogen induced mammary carcinogenesis in ovariectomized W/Fu rats. Jpn J Cancer Res. 1985;76(8):699–704.

5. Barrat J, de Lignieres B, Marpeau L, et al. Effect in vivo de l’adminstration locale de progesterone sur l’activite mitotique des glaactorphores humains. [The in vivo effect of the local administration of progesterone on the mitotic activity of human ductal breast tissue. Results of a pilot study.] J Gynecol Obstet Biol Reprod (Paris). 1990;19(3):269–274.

6. Malet C, Spritzer P, Guillaumin D, Kuttenn F. Progesterone effect on cell growth, ultrastructural aspect and estradiol receptors of normal breast epithelial (HBE) cells in culture. J Steroid Biochem Mol Biol. 2000;73(3–4):171–181.

7. Mauvais-Jarvis P, Kuttenn F, Gompel A. Antiestrogen action of progesterone in breast tissue. Breast Cancer Res Treat. 1986;8(3):179–188.

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