Do bio-identical hormones cause breast cancer?

Dr. Frank Shallenberger, MD

December 7, 2020



Marie is a 49-year-old woman, who has always taken great care of herself. She eats healthy foods, avoids over-indulgence, gets to bed early, and exercises regularly. She feels and looks great. There’s only one problem.

Marie is starting to go through menopause. She came into the office last week doing something I’ve never heard her do — complaining.

She was complaining of being tired, feeling depressed, and not being able to sleep well. So I told her all she had to do to feel her usual self was to replenish her depleting hormones with bio-identical ones. That’s when a worried look came over her face.

She then told me about the same concern that I have heard from so many other women in her position: Don’t they cause cancer?

“But I’ve heard that the bio-identical hormones are no safer than the synthetic ones. I just don’t feel comfortable taking them until studies prove their safety.”

Fifteen years ago, I would have admitted: “There are no published studies on the safety of bio-identical hormones. Both the FDA and The Endocrine Society have said that they are just as dangerous as synthetic hormones.”

But bio-identical hormones are very different from the synthetic hormones that cause disease. They are molecularly identical to the hormones that are already in your body, and have been there all your life. If they have not harmed you for this long, why would they suddenly have a different effect?

What Recent Studies Have Found

And, just as importantly, researchers recently conducted a study on the safety of bio-identical hormones. They even published it in the prestigious cancer journal, The Journal of Clinical Oncology.

The results of this study should alleviate the concerns that Marie and so many other women have about the safety of bio-identical hormones.

This study isn’t some small, statistically unsound study. It’s a study of over 80,000 postmenopausal nurses. And it covers over eight years of follow up.

The authors of the study divided the nurses into three groups: those taking synthetic hormones, those taking bio-identical hormones, and those taking no hormones. Then they compared the three groups in terms of who developed breast cancer the most, and what types of breast cancer they developed.

Here’s What They Found

The women taking the synthetic hormones had a 60% greater chance of developing ductal breast cancer than women who took no hormones. They had twice the risk of developing lobular breast cancer. And they had almost four times the risk of developing a mixed ductal/lobular form of cancer!

Compare this to the women who took bio-identical hormones. They had the same cancer risk in all categories as the women who took no hormones. In other words, the women in this large study who took bio-identical hormones had no more risk of developing breast cancer than the women who took no hormones at all!

Plus, they felt better. They functioned like they were years younger. And they decreased their risk of Alzheimer’s, osteoporosis, diabetes, heart disease, and strokes. They had all of the benefits of having youthful levels of hormone, but without any increased risk of breast cancer.

So if you have been denying yourself all of the many advantages of replenishing your deficient hormone levels with bio-identical ones out of concern for breast cancer, it’s time to reconsider. I hope, like Marie, you will now feel convinced about their safety. 


Fournier, A., A. Fabre, S. Mesrine, et al. “Use of different postmenopausal hormone therapies and risk of histology- and hormone receptor-defined invasive breast cancer,” J Clin Oncol. 2008 March; 26(8): 1260-1268.

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