Do Hormones Really Cause Cancer?

One of the first things many women ask me when we talk about hormone replacement therapy (HRT) is:
“Don’t hormones cause cancer?”

This fear has been around for a long time, but it’s mostly based on old and misunderstood research. As menopause specialists, we now have a better understanding—and no, we do not believe that hormones cause cancer when used appropriately.

Where Did the Myth Come From?

The fear began in 2002, when the first results from a large research study called the Women’s Health Initiative (WHI) were released. The study actually started in the early 1990s and looked at how HRT affected risks for things like heart disease, cancer, and osteoporosis.

There were two groups in the study:

Estrogen + Progestin: for women who still had a uterus
Estrogen Alone: for women who had a hysterectomy

In 2002, researchers published 5 year data that showed women in the Estrogen + Progestin group had a small increase in breast cancer. This made headlines—and many women immediately stopped using HRT.

But there’s more to the story.

What the Data Actually Showed

That reported increase in breast cancer risk was very small, and it wasn’t statistically significant. There’s also a chance it was due to detection bias—meaning the researchers were looking extra closely for cancer in that group.

They also reported the relative risk (how much more common breast cancer was in one group vs. another) instead of the absolute risk (how many actual cases happened). When you look at the absolute numbers, the increase was just 0.08% per year, or 8 more cases of breast cancer per 10,000 women each year.

Important Study Limitations

There were other issues with the WHI study:

The average age of the women was 63. Many were much older than the age we now recommend starting HRT (within 10 years of menopause).
The progestin used was medroxyprogesterone acetate (MPA), a synthetic hormone. Today, we prefer to use bioidentical hormones, which are more similar to the body’s natural hormones and may be safer.

What Affects Breast Cancer Risk?

If you’re worried about breast cancer, it’s important to know that lifestyle factors also play a big role—possibly more than HRT. These include:

Having a BMI over 30 (obesity)
Not getting enough physical activity
Drinking alcohol daily
Smoking (or past smoking)

The American Cancer Society estimates that the average woman has about a 13% lifetime risk of breast cancer. But if you’re postmenopausal and obese, your risk may go up by 30–40%, raising your lifetime risk to around 17–18%.

In the WHI study:

28% of non-Hispanic white women and 50% of non-Hispanic Black women had a BMI over 30
6.2% were current smokers, and 42.9% were former smokers

Because of their older age and higher rates of obesity and smoking, many WHI participants already had a higher risk of breast cancer—before HRT was even added.

The Benefits of HRT

Despite the confusion around breast cancer, HRT showed many clear benefits in the WHI study:

Stronger bones and fewer fractures
Lower risk of colon cancer
Lower risk of heart disease (especially in younger women)
Lower all-cause mortality (death from any cause)
Relief from menopausal symptoms like hot flashes, night sweats, and vaginal dryness

The fear that “hormones cause cancer” came from misinterpreted data, not proven facts. When used at the right time and in the right way, HRT is safe and effective for many women. It can help you feel better, live healthier, and age more confidently.

If you’re unsure whether HRT is right for you, talk to a menopause-trained healthcare provider who can look at your personal risk factors and help you make an informed choice.

References:

Calle, E. E., Rodriguez, C., Walker-Thurmond, K., & Thun, M. J. (2003). Overweight, obesity, and mortality from cancer in a prospectively studied cohort of U.S. adults. The New England journal of medicine, 348(17), 1625–1638. https://doi.org/10.1056/NEJMoa021423

Chen, W. Y., Rosner, B., Hankinson, S. E., Colditz, G. A., & Willett, W. C. (2011). Moderate alcohol consumption during adult life, drinking patterns, and breast cancer risk. JAMA, 306(17), 1884–1890. https://doi.org/10.1001/jama.2011.1590

Gaudet, M.M., Gapstur, S.M., Sun, J., Diver, W.R., Hannan, L.M., and Thun, M.J. (2013). Active smoking and breast cancer risk: Original cohort data and meta-analysis. Journal of the National Cancer Institute, 105(8), 515–525, https://doi.org/10.1093/jnci/djt023

Pizot C, Boniol M, Mullie P, et al. (2016). Physical activity, hormone replacement therapy and breast cancer risk: A meta-analysis of prospective studies. European Journal of Cancer, 52, 138-154.

Women’s Health Initiative (WHI). (n.d.). About WHI. Retrieved from https://www.whi.org/about-whi.