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Research on Hormone Replacement Therapy and Breast Cancer Risk


Numerous studies have looked at the potential health benefits and risks of hormone replacement therapy — particularly the increased risk of breast cancer. However, it’s hard to compare their results and make sense of them all together, because these studies often differ in key ways: the type of estrogen/progestogen used; the duration of use; the dosage; the age at which participating women started HRT; and other health characteristics of participants. For the medical community, piecing it all together is like taking puzzle pieces from different boxes and trying to form them into a picture that makes sense. Also, when you see a news report about HRT, it’s usually based on the conclusions of the latest study, rather than the body of research as a whole. That can make things even more confusing.

Hormone replacement therapy is controversial and complex, but for now it is generally not recommended for women with a history of breast cancer, or those at higher-than-average risk. HRT is also not recommended if you’ve had ovarian or uterine cancer, a history of blood clots, and conditions affecting the heart, such as a past heart attack or evidence of coronary artery disease (blockages in vessels that feed blood to the heart).

Here’s what the research appears to show now — but keep in mind that these results aren’t “final.”

For combination HRT (estrogen and progestogen), there is conflicting data about the short-term and long-term risk of breast cancer in the general population. The most recent results of a long-term study known as the Women’s Health Initiative suggest that, after using combination HRT for 5 years, women had a slightly increased risk of breast cancer, coronary heart disease, stroke, and venous thromboembolism (blood clots in the veins). The small increased risk of breast cancer persisted even after women stopped taking combination HRT. The women had a decreased risk of fractures and colon cancer.

As for combination HRT use in women with a previous breast cancer diagnosis, there have only been a few studies done. The largest one, called the HABITS trial (Hormonal Therapy After Breast Cancer: Is It Safe?”), was stopped early because it was linked with an increased risk of breast cancer recurrence.

For estrogen-only HRT, there is also conflicting data regarding both short-term and long- term risk of breast cancer in the general population. Most recently, the results from the long-term Women’s Health Initiative study linked estrogen-only HRT to a decreased risk of breast cancer and an increased risk of venous blood clots and stroke, but not an increased risk of cardiovascular events. 

For women at higher-than-average risk of breast cancer, there are only small studies of combination HRT and estrogen-only HRT use and more research needs to be done. It is not known whether combination HRT or estrogen-only HRT increases the risk of eventually developing breast cancer. However, there are concerns that it could, and right now the risks of combination HRT are believed to outweigh the benefits.

— Last updated on February 7, 2022, 7:00 PM