It depends on the type of hormone therapy. In the largest randomized trial, estrogen plus progestin caused a small increase in breast cancer, while estrogen alone did not, and was linked to fewer breast cancers over long-term follow-up. Your own risk depends on the type, dose, how long you use it, when you start, and your personal history.
That answer is less frightening than the headlines many women remember, and more nuanced than “hormones are safe.” Here is what the evidence actually says.
Where the fear came from: the Women’s Health Initiative
The Women’s Health Initiative (WHI) is the study behind most of today’s worry. It ran two separate randomized trials, and they gave different answers for breast cancer.
The estrogen plus progestin trial
This trial enrolled 16,608 postmenopausal women aged 50 to 79 who still had a uterus (Rossouw et al., JAMA, 2002). They took conjugated equine estrogens plus medroxyprogesterone acetate, or a placebo.
It was stopped in 2002 after an average of 5.2 years, because invasive breast cancer crossed a preset safety boundary. In absolute terms, the researchers estimated 8 more invasive breast cancers per 10,000 women per year of use. That is a real increase, and also a small one for any individual woman.
The estrogen-alone trial
This trial enrolled 10,739 women who had already had a hysterectomy (Anderson et al., JAMA, 2004). They took conjugated equine estrogen alone, or a placebo. Estrogen alone did not increase breast cancer during the trial; the authors said a possible reduction needed further study.
Estrogen alone is only used for women without a uterus. The National Cancer Institute explains why: estrogen without a progestin raises the risk of endometrial (uterine) cancer, so women with a uterus are generally prescribed both.
What 20 years of follow-up showed
Long-term follow-up confirmed the split between the two types. A 2020 analysis followed the WHI participants for more than 20 years (Chlebowski et al., JAMA, 2020).
- Estrogen plus progestin: higher breast cancer incidence than placebo (hazard ratio 1.28), with no statistically significant difference in breast cancer deaths.
- Estrogen alone: lower breast cancer incidence than placebo (hazard ratio 0.78) and lower breast cancer deaths (hazard ratio 0.60).
The National Cancer Institute adds two practical points. The higher breast cancer risk from prior estrogen plus progestin use lasted at least a decade after stopping, and combined therapy increased breast density, which makes mammograms harder to read (NCI fact sheet).
Why age and timing matter
The women in WHI had an average age of 63.4 at enrollment (Chlebowski et al., 2020). That is older than the under-60 group that current guidance treats differently.
The 2022 Hormone Therapy Position Statement from The Menopause Society, then called The North American Menopause Society, spells out why timing matters (Menopause, 2022). Its key conclusions:
- The risks of hormone therapy differ by type, dose, duration of use, route of administration, timing of initiation, and whether a progestogen is used.
- For women younger than 60 or within 10 years of menopause onset, without contraindications, the benefit-risk ratio is favorable for treating bothersome hot flashes and night sweats and for preventing bone loss.
- For women who start after age 60 or more than 10 years past menopause, the balance appears less favorable, mainly because of higher absolute risks of heart disease, stroke, blood clots and dementia.
- Treatment should be individualized, with periodic reevaluation of whether to continue.
Cleveland Clinic summarizes the same idea for patients: risks are generally lower when hormone therapy starts before age 60 or within 10 years of menopause.
Your personal history changes the answer
Population averages are a starting point, not your answer. Your own history can raise or lower the stakes considerably.
The American College of Obstetricians and Gynecologists (ACOG) describes combined hormone therapy as carrying a small increased risk of breast cancer, and advises that women with a history of hormone-sensitive breast cancer try nonhormonal treatments first. Cleveland Clinic lists a current or past breast, ovarian or uterine cancer, abnormal vaginal bleeding, blood clots, and a history of stroke or heart attack among the reasons a provider may decide hormone therapy is not right for you.
Things worth bringing to any conversation about hormone therapy:
- Whether you still have a uterus
- Any personal history of breast, ovarian or uterine cancer, blood clots, stroke or heart disease
- Breast cancer in close relatives
- Your most recent mammogram date and result
- How many years it has been since your last period
Questions to ask before deciding
The useful question is not “is it safe?” but “what is my risk, compared with my benefit, for this specific option?” A good evaluation answers that for you.
- Which type of therapy fits my situation, and why?
- What is my baseline breast cancer risk?
- How long would we plan to use it, and how often would we reassess?
- What nonhormonal options exist for my symptoms?
If you are weighing these questions, our women’s hormone evaluation is a place to talk through your symptoms, history and options with a provider. It is an evaluation pathway first: the goal is to understand what is going on and whether any treatment is appropriate, not to assume that it is.
What an appointment at Hormone Balance Centers looks like
Hormone Balance Centers is a telehealth practice for eligible adults in Utah and Idaho. Visits are by secure video, with evening appointments available, and women’s hormone evaluation is a separate pathway from men’s care.
- Call or text 385-361-8549.
- Labs. Your provider decides which tests, if any, are appropriate.
- Meet online with Adam Russell, PA-C, a board-certified Physician Assistant with experience in endocrinology and internal medicine, to review your symptoms, history, labs, risks and goals.
- A plan. If treatment is appropriate, you get an individualized care and monitoring plan. The practice prescribes only when it is medically appropriate.
Care is self-pay. Insurance is not billed directly, but you can request documentation for possible out-of-network reimbursement.
Frequently asked questions
Does estrogen alone cause breast cancer?
In the WHI trial of women without a uterus, estrogen alone did not increase breast cancer, and long-term follow-up found fewer breast cancers and fewer breast cancer deaths than placebo. Estrogen alone is only appropriate for women who have had a hysterectomy, because it raises uterine cancer risk.
How much does combined hormone therapy raise breast cancer risk?
In the WHI estrogen plus progestin trial, researchers estimated about 8 additional invasive breast cancers per 10,000 women per year of use (Rossouw et al., JAMA, 2002). That increase is real but small for an individual, and it varies with dose, duration, timing and personal risk factors.
Is hormone therapy safer if I start it earlier?
For healthy women under 60 or within 10 years of menopause, The Menopause Society considers the benefit-risk ratio favorable for hot flashes and bone protection. Starting later appears less favorable. Breast cancer risk is one part of that balance, alongside heart, stroke and clot risk.
Can I use hormone therapy if breast cancer runs in my family?
That decision needs an individual evaluation. Family history changes your baseline risk, and ACOG advises women with a personal history of hormone-sensitive breast cancer to try nonhormonal options first. Only a full review of your history can say what is reasonable for you.
Get your questions answered
Every woman’s risk picture is different, and the right answer starts with your history, not a headline. Call or text 385-361-8549, read about our women’s hormone evaluation, or get started online.
Sources
- Rossouw JE, et al. Risks and benefits of estrogen plus progestin in healthy postmenopausal women: principal results from the Women’s Health Initiative randomized controlled trial. JAMA. 2002;288(3):321-333. https://pubmed.ncbi.nlm.nih.gov/12117397/
- Anderson GL, et al. Effects of conjugated equine estrogen in postmenopausal women with hysterectomy: the Women’s Health Initiative randomized controlled trial. JAMA. 2004;291(14):1701-1712. https://pubmed.ncbi.nlm.nih.gov/15082697/
- Chlebowski RT, et al. Association of Menopausal Hormone Therapy With Breast Cancer Incidence and Mortality During Long-term Follow-up of the Women’s Health Initiative Randomized Clinical Trials. JAMA. 2020;324(4):369-380. https://pmc.ncbi.nlm.nih.gov/articles/PMC7388026/
- The 2022 Hormone Therapy Position Statement of The North American Menopause Society. Menopause. 2022;29(7):767-794. https://pubmed.ncbi.nlm.nih.gov/35797481/
- National Cancer Institute. Menopausal Hormone Therapy and Cancer. https://www.cancer.gov/about-cancer/causes-prevention/risk/hormones/mht-fact-sheet
- American College of Obstetricians and Gynecologists. Hormone Therapy for Menopause. https://www.acog.org/womens-health/faqs/hormone-therapy-for-menopause
- Cleveland Clinic. Hormone Replacement Therapy (HRT) for Menopause. https://my.clevelandclinic.org/health/treatments/15245-hormone-therapy-for-menopause-symptoms