BHRT and breast cancer: facts, figures, and what the science really says

Does hormone therapy raise the risk of breast cancer? For most women with no history of breast cancer, the absolute risk is small, but not zero. How large it is depends on the type of hormone therapy, how long you use it and your personal risk profile. Combined therapy, oestrogen together with a progestogen, is linked to a slightly higher risk than oestrogen alone, and that risk rises the longer you continue. This article sets out the facts, honestly and grounded in the evidence, without downplaying the risks and without unnecessary fear.

In short

• The familiar fear comes largely from the 2002 WHI study, which examined oral, synthetic hormones in women with an average age of 63, not transdermal bioidentical hormone therapy.
• Hormone therapy does have a real, measurable effect: in large studies almost every form of combined therapy has been linked to a small extra risk that increases with duration of use and can persist for years after stopping.
• The type of progestogen appears to matter. For bioidentical (micronised) progesterone some data point to a lower risk than synthetic progestogens, but the evidence is limited and not conclusive.
• The absolute risk stays small. In the WHI it amounted to fewer than one extra case per 1,000 women per year, comparable to everyday lifestyle factors such as two glasses of alcohol a day.
• Always discuss your personal risk profile, including family history and any BRCA mutations, with a doctor, and keep taking part in breast cancer screening regardless of your treatment.

The facts at a glance

What the current evidence does and does not show:

• What it does show: almost all forms of hormone therapy except local (vaginal) oestrogens have been linked to a small extra risk of breast cancer in a large international meta-analysis (Lancet, 2019). Combined therapy carries more risk than oestrogen alone, and longer use more than short use.
• What is less certain: whether bioidentical progesterone and the transdermal route genuinely lower that risk. Some data point that way, but authoritative guidelines consider the evidence still too limited for a firm conclusion.
• What it does not show: that hormone therapy causes breast cancer in every user, or that the absolute risk is large for most women. It is a small shift in probability, weighed against the benefits of treatment.

Where the fear comes from: the 2002 WHI study

In 2002 the Women's Health Initiative (WHI) published a study that caused a stir: women using hormone therapy had an increased risk of breast cancer. Doctors stopped prescribing it, many women stopped their treatment, and the fear that emerged then persists to this day.

The study had limitations that strongly affect how it translates to everyday practice:

• Participants were on average 63 years old and started hormone therapy ten to twelve years after menopause.
• They used oral oestrogen (conjugated equine oestrogens) combined with the synthetic progestogen MPA (medroxyprogesterone acetate).
• It did not involve bioidentical hormones given transdermally.
• A portion of the participants already had an elevated baseline risk of breast cancer before the study.

Importantly, for breast cancer the finding was of borderline statistical significance, a roughly 26 percent relative increase with a confidence interval that reached the no-effect line. The results apply mainly to that specific combination in that specific group, and do not automatically apply to transdermal bioidentical hormone therapy (BHRT) in women who start early.

What science has shown since then

The type of progestogen appears to matter. This is one of the most consistent findings since 2002. Synthetic progestogens, such as MPA, have been linked to an increased breast cancer risk in several studies. For bioidentical progesterone some data point to a lower risk, but this is not settled.

A large French cohort study (E3N) found no clear increase in risk for oestrogen combined with bioidentical progesterone in the first years of use, while synthetic progestogens in the same study did raise the risk (Fournier, 2008). These French data were not included in the Lancet meta-analysis, and with longer use the picture becomes less certain. NICE, the UK guideline authority, concludes there is insufficient evidence to say the risk with bioidentical progesterone is definitely different from other progestogens.

The route of administration mainly matters for blood clots. Transdermal estradiol, through the skin, has a more favourable profile for clot risk than oestrogen in tablet form. For breast cancer the risk is driven mainly by the progestogen and the duration of use, and oestrogen alone carries a smaller risk than combined therapy.

Timing counts. Starting early, within ten years of menopause, has a more favourable balance of benefits and risks. This is also the window in which hormone therapy tends to show the most benefit for the heart and bones.

Unsure whether hormone therapy fits your situation? A sound decision starts with good information about menopause and your personal risk profile. Download the free guide →

The absolute risk in perspective

Even in the WHI, using the least favourable type of hormone therapy, the absolute risk was fewer than one extra case of breast cancer per 1,000 women per year. That is comparable to the risk of two glasses of alcohol a day, or a BMI over 30.

The Lancet meta-analysis makes it more concrete: five years of combined therapy from age 50 is linked to about one extra case of breast cancer per fifty users, counted across ages 50 to 69. For oestrogen alone it is roughly one per two hundred. Longer use has a larger effect, and part of the raised risk persists for years after stopping.

This does not mean risk is unimportant. It means keeping it in proportion. Women weigh risks every day in diet, exercise and alcohol without dramatising them. The risk of untreated, severe menopausal symptoms, for bone density, heart, cognition and quality of life, is also real and measurable.

When is extra caution needed?

There are situations where hormone therapy is not a given and calls for careful discussion:

• Active breast cancer: systemic hormone therapy is contraindicated during active treatment.
• History of hormone receptor-positive breast cancer: the most complex situation, always requiring consultation with an oncologist. Local oestrogen therapy for vaginal symptoms is sometimes possible.
• Strongly elevated familial or genetic risk (BRCA1/2): a relative contraindication. The decision is individual and calls for genetic counselling.

Women without a history of breast cancer and without a strongly elevated genetic risk do not need to decline hormone therapy on fear alone. For them the risk profile of transdermal estradiol with bioidentical progesterone is relatively favourable, though a personal assessment with a doctor remains necessary.

When to see your (GP) doctor?

Always contact a doctor if you feel a lump or change in your breast, notice unusual nipple discharge or skin changes, or are worried about your personal risk. This is independent of hormone therapy. Also have a conversation before starting or stopping hormone therapy, so your choice fits your risk profile and symptoms.

Hormone therapy and breast cancer screening

Using hormone therapy is no reason to skip population screening (mammography), on the contrary. Regular screening is just as important for users as for women who do not use hormone therapy. Hormone therapy can slightly increase breast tissue density, which makes a mammogram a little harder to interpret. Always mention that you use hormone therapy so the radiologist can take it into account.

Talking to your doctor

The decision about hormone therapy is always individual. Your personal risk profile, family history, age, BMI, lifestyle and the severity of your symptoms together shape the balance. A doctor specialising in hormonal health can weigh these factors with you based on current science.

At Menovia we discuss breast cancer risk as a standard part of the intake, honestly and fully. If there are contraindications, we say so. If hormone therapy fits your risk profile and is appropriate, we explain why. Read more about our approach to hormone therapy.

Frequently asked questions

Does all hormone therapy raise breast cancer risk? Not to the same degree. Local (vaginal) oestrogens show no increased risk. Oestrogen alone carries a small risk, combined therapy a slightly higher one, rising with duration of use.

Is bioidentical progesterone safer than synthetic progestogens? Some data suggest a lower risk with bioidentical progesterone, but the evidence is limited. Authoritative guidelines say it is insufficient to confirm a definite difference.

Does the risk persist after stopping? Part of the raised risk can persist for years after stopping. How much depends on how long you used hormone therapy.

I have had breast cancer. Can I use hormone therapy? With a history of hormone receptor-positive breast cancer, systemic hormone therapy is usually not advised. This always requires consultation with your oncologist. For vaginal symptoms, local therapy is sometimes possible.

Do I need more frequent checks if I use hormone therapy? At the very least, keep taking part in routine screening. Discuss with your doctor whether additional monitoring makes sense in your situation.

Sources

• Writing Group for the Women's Health Initiative Investigators. Risks and Benefits of Estrogen Plus Progestin in Healthy Postmenopausal Women. JAMA, 2002. pubmed.ncbi.nlm.nih.gov.
• Collaborative Group on Hormonal Factors in Breast Cancer. Type and timing of menopausal hormone therapy and breast cancer risk. The Lancet, 2019. pubmed.ncbi.nlm.nih.gov.
• Fournier A, et al. Unequal risks for breast cancer associated with different hormone replacement therapies (E3N cohort). Breast Cancer Research and Treatment, 2008. pmc.ncbi.nlm.nih.gov.
• NICE. Menopause: identification and management (NG23). National Institute for Health and Care Excellence. nice.org.uk.

This information is intended as an educational resource and does not replace medical advice, diagnosis, or treatment. If you have symptoms or questions about your personal situation, we recommend consulting a doctor.