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Treatments primer · HRT & cancer

HRT and cancer risk, in real numbers.

The conversation everyone wants to skip and nobody should. What HRT actually does to your risk of breast, endometrial, ovarian and other cancers, by formulation and duration, in absolute numbers rather than scary headlines. Education only; the decision is yours and your doctor or specialist's.

Why this page exists.

Most cancer-risk conversations about HRT are either alarmist (one number out of context) or dismissive ("the risk is tiny, don't worry"). Both cheat you out of the actual information. Here's the honest version: cancer by cancer, with the absolute numbers where they exist, the caveats that matter, and links to the fuller story.

If you've already had cancer and are navigating menopause alongside treatment, the page you want is Pathway: Menopause after cancer.

And if your question is "can I take this supplement / herb / peptide / medication with my cancer treatment?" — that's a different tool: Is this safe with my cancer treatment? (interaction checker).

Breast cancer

Small absolute increase with combined HRT after several years; estrogen-only HRT (no uterus) appears to slightly lower breast cancer incidence and mortality. Type of progestogen matters: micronized progesterone and dydrogesterone carry a lower signal than older synthetic progestins.

The most studied and most feared number. The honest, current version: combined HRT (estrogen + a progestogen) is associated with a small absolute increase in breast cancer risk that grows with duration of use, mostly after about 5 years, and tapers off — but doesn't fully disappear — for about a decade after stopping (Lancet 2019 meta-analysis of 58 studies). Estrogen-only HRT in women without a uterus did the opposite in the Women's Health Initiative (WHI): at 20-year follow-up it was associated with a roughly 22% lower incidence of breast cancer and a 40% lower breast cancer mortality (Chlebowski, JAMA 2020). Type of progestogen matters more than the older WHI data could show: the UK QResearch/CPRD nested case-control of nearly 100,000 women (Vinogradova, BMJ 2020) found micronized progesterone and dydrogesterone carried a smaller signal than norethisterone, medroxyprogesterone acetate (MPA) or levonorgestrel — though no oral progestogen was completely neutral over many years. NICE updated its UK menopause guideline (NG23, November 2024) to reflect this nuance and to present risk in absolute, baseline-adjusted numbers rather than relative risk alone.

The numbers

WHI combined-HRT arm (conjugated equine estrogen + MPA): about 8 extra breast cancer cases per 10,000 women per year — smaller than the risk attributed to drinking 2 glasses of wine a night, being 5 BMI units heavier, or current smoking. WHI estrogen-only arm (no uterus): about 7 fewer cases per 10,000 women per year and a statistically significant reduction in breast cancer mortality at 20-year follow-up. Vinogradova/BMJ 2020 (per 10,000 women aged 50–59 treated for 5 years, above a baseline of ~27 cases): micronized progesterone +5, dydrogesterone +9, MPA +9, norethisterone +13, levonorgestrel +14. After stopping HRT, residual risk decays over roughly 10 years.

Sources

Endometrial (uterine) cancer

Estrogen alone increases risk if you have a uterus. Adding adequate progesterone removes the increase.

This is the cleanest of the cancer-risk conversations. Unopposed estrogen (estrogen without progesterone) thickens the uterine lining and significantly raises endometrial cancer risk. The whole reason combined HRT exists is to prevent that. If you have a uterus and you're on estrogen, you need adequate progesterone (or a Mirena IUS, which delivers it locally). If you don't have a uterus, this concern doesn't apply to you.

The numbers

Unopposed estrogen for 5+ years: roughly 5-10x baseline endometrial cancer risk. Combined HRT with adequate progesterone: no meaningful increase, and possibly a small decrease.

Sources

Ovarian cancer

A very small absolute increase with long-term HRT use, observed mostly in long-term studies.

The signal here is real but small, and seen across both combined and estrogen-only HRT in observational data. The Collaborative Group's 52-study reanalysis (Lancet 2015) put the absolute increase at roughly 1 extra ovarian cancer per 1,000 women using HRT for 5 years from age 50, and about 1 extra death per 1,700. The signal attenuates after stopping but persists longer than the breast signal. For women with a strong family history or a BRCA1/2 variant — and especially after risk-reducing salpingo-oophorectomy, where HRT is often actively recommended up to the natural age of menopause — this is a personalised conversation; UK and US guidance broadly supports HRT in that setting because the risk of premature surgical menopause outweighs the small ovarian risk.

Sources

Colorectal cancer

HRT slightly reduces risk, especially with combined formulations.

One of the underreported parts of the WHI: combined HRT reduced colorectal cancer incidence in the trial. The mechanism isn't fully understood. This isn't a reason to start HRT for colorectal protection (screening is what does that work), but it's part of the honest accounting when people only quote the breast cancer side of the ledger.

Sources

Lung cancer

No clear increase from HRT; smoking remains by far the dominant risk factor.

Some early signals around HRT and lung cancer mortality in smokers haven't held up consistently. The lung cancer risk that matters is smoking history. If you smoke, the smoking conversation is the bigger one by orders of magnitude.

Sources

Vaginal estrogen and cancer risk

Local vaginal estrogen has minimal systemic absorption and is considered safe for most women, including most breast cancer survivors after individualized conversation.

The blanket 'no estrogen ever' rule that many breast cancer survivors are given for vaginal symptoms is more nuanced than it sounds. Vaginal estrogen at standard low doses delivers a tiny fraction of the systemic dose. The largest cohort to date — a Danish nationwide study of more than 8,000 breast cancer survivors using vaginal estrogen (Cold et al., JNCI 2022) — did not find an overall increase in breast cancer recurrence, with the exception of a subgroup on aromatase inhibitors where the picture was less clear. The Menopause Society, the British Menopause Society, NICE (NG23, 2024) and ACOG all support its use in many breast cancer survivors when genitourinary syndrome of menopause (GSM) is severely affecting quality of life, after non-hormonal options (moisturisers, lubricants, vaginal DHEA, laser) have been tried and after a conversation with the oncology team. This is a 'talk to your specific oncologist' answer, not a website answer.

Sources

One last honest sentence about all of this.

"HRT and cancer" is almost never one number; it's a personal calculation that depends on which HRT, what dose, what formulation, how long, your age when you start, your family history, your other risk factors and what untreated symptoms are doing to your life. A doctor or specialist who talks about it as a conversation rather than a verdict is the one to keep.

For the broader picture on how HRT got mis-sold in both directions, HRT myths, honestly answered pairs with this page.

Already had cancer?

Menopause after cancer

This page is about deciding whether to start HRT. If you've already been treated for a hormone-sensitive cancer, the clinical pathway you want lives here.

Open the pathway

Research companion

Is this safe with my cancer treatment?

Look up supplements, herbs and medications against your cancer profile (breast, endometrial, ovarian). Cited. Conservative. Walks you into the next oncology appointment prepared.

Open the checker
Back to treatments primerQuestions to bring to your doctor or specialist

References & further reading

Where the numbers on this page come from

Breast-cancer and HRT risk gets distorted both ways — overstated by some, understated by others. These are the source studies and guidelines, in their own words.

Primary sources

Type and timing of menopausal hormone therapy and breast cancer risk

Collaborative Group on Hormonal Factors in Breast Cancer, Lancet 2019

The largest pooled analysis to date. The paper most modern risk estimates trace back to.

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Long-term influence of CEE and CEE + MPA on breast cancer incidence and mortality

Chlebowski et al., JAMA 2020 (WHI long-term)

The WHI follow-up showing estrogen-alone HRT was associated with lower breast-cancer mortality — the finding that rarely makes the headlines.

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NICE patient decision aid — long-term benefits and risks of HRT

NICE (UK)

The risk table in absolute numbers per 1,000 women, formatted for patients. The clearest single page in print.

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The 2022 Hormone Therapy Position Statement

The Menopause Society (NAMS)

The North American consensus document. Covers cancer, cardiovascular, and bone outcomes in the same place.

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