What you may have heard
Women who take hormone replacement therapy have a higher risk of developing breast cancer.
What science tells us
There are various types of hormone replacement therapy (HRT), including estrogen only, estrogen + progestin (combined), or bioidentical hormone replacement therapies.
Estrogen. While hormone replacement therapy with estrogen alone improves symptoms of menopause and may decrease risk of breast cancer in people with no family history of breast cancer, it increases the risk of endometrial (lining of the uterus) cancer (ACS). The risk of endometrial cancer remains higher than normal even after the therapy has stopped.
Estrogen + progestin (combined). You may also treat menopausal symptoms with estrogen and progestin therapy, which is known as combined hormone therapy. The longer the HRT is used, the higher the risk. The risk returns to normal within three years of stopping the hormones, so there is a benefit to stopping HRT if a woman has already started the treatment.
Bioidentical hormone therapy. This hormone therapy uses hormones that contain estrogen and progesterone with the same chemical structures as those found naturally in the body.
The progestin in the combined hormone therapy lowers the risk of endometrial cancer that would be conferred by estrogen. However, the Women’s Health Initiative study showed that women who use combined hormone therapy have a higher risk of breast cancer and ovarian cancer (NCI). The Million Women Study similarly found an increase in breast cancer risk for the combination estrogen and progestin therapy (Clarkson et al.).
While the bioidentical hormone therapy is marketed as a safer alternative, no studies have found that women taking bio-identical hormones have less serious side effects (NCI). Bioidentical hormones have the same health risks as any other type of hormone therapy.
Toxicology Evidence/Supporting Evidence
Studies that have examined the impact of HRT on monkeys and mice have shown that administration of estrogen + progestin increased the risk of breast cancer (Clarkson et al.)
IARC Carcinogen Classification: Group 1 (carcinogenic to humans)
How to reduce your risk
Any decision to use estrogen, either alone or in combination with a progestin therapy, should be made by each individual woman and her doctor after weighing risks and benefits (ACS). Some things to consider are baseline risk of breast, endometrial, and ovarian cancer (and how this may be affected by hormone therapy); risk of other serious conditions affected by hormone therapy (example: heart disease, stroke, serious blood clots, and effects on the brain); and what other medicines may be used to treat osteoporosis or menopausal symptoms. If the decision is made to take HRT, it is best to use it at the lowest dose needed, for as short a time as possible, and to continue seeing your doctor regularly so they may monitor you for side effects (ACS).
Note: All women should report any vaginal bleeding that happens after menopause right away; it may be a sign of endometrial cancer. Even if your risk is not elevated, you still have a chance of getting endometrial cancer.
HRT may increase your risk of breast, endometrial, and/or ovarian cancer. Speak to your doctor before making a decision about hormone replacement therapy.
Learn More From These Trusted Sources
American Cancer Society (ACS): HRT
National Cancer Institute (NCI): HRT
MD Anderson: Hormones and cancer
Published: June 10, 2021
Verified/updated: August 22, 2022