Estrogen Therapy During Menopause: Does It Increase or Reduce Breast Cancer Risk?

But should this be routine for every postmenopausal woman? This is an issue requiring careful discussion in light of scientific evidence . In this article, we examine estrogen therapy’s effects on breast cancer risk, focusing on research published on April 23, 2024 in Breast Cancer Research and Treatment as a recent meta-analysis of the available evidence.

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🔍 A Light Amid Conflicting Findings: Does Estrogen Alone Reduce Breast Cancer Risk?

A long-debated question: Does estrogen-alone therapy (without progestin) increase or reduce breast cancer risk?

While observational studies mostly suggest an increase, the most prominent randomized controlled trial, Women’s Health Initiative (WHI) , showed the opposite: a significant reduction in risk with treatment.

📊 A New Meta-Analysis: More Than 14,000 Women, Strong Data

Rowan Chlebowski and colleagues conducted the study to demonstrate the actual effect of estrogen alone on breast cancer incidence (annual new cases) . We explore the details below.

📘 Meta-Analysis Summary

Published in 2024, this meta-analysis included WHI (Women’s Health Initiative) among 10 randomized controlled trials examining how estrogen-alone hormone therapy affects breast cancer risk.

📊 Main Findings – What Do They Mean?

• 🔹 Across the 9 smaller trials:
  Among women receiving estrogen, only 1.2% developed breast cancer, compared with 2.2% in the placebo group.
  👉 This suggests estrogen may reduce risk by approximately 35% (although the difference is not statistically conclusive).
• 🔹 In 5 estradiol (a type of estrogen) trials:
  Only the risk ratio was reported: Breast cancer risk was 37% lower with treatment.
  👉 This result is also borderline; more data may be needed for a firm conclusion.
• 🔹 Pooled results from all studies:
  Breast cancer occurred in 3.6% of estrogen-treated women versus 4.7% in the placebo group.
  👉 Overall, estrogen therapy therefore reduced breast cancer risk by 23%.
  📌 This difference was statistically significant (P = 0.002), meaning its likelihood of arising by chance was very low.

🤔 Why Is It So Controversial?

Because some large observational cohorts (such as the Million Women Study) report the opposite: a 22% increase in breast cancer risk with long-term estrogen use was observed.

This new meta-analysis, however, includes only randomized controlled trials, regarded as providing stronger evidence than observational studies in this context.

What might explain the difference?

• In randomized trials, estrogen is mostly given to women who have had a hysterectomy ; risk may be lower because no progestin is added.
• All WHI participants received regular mammograms, reducing delayed-diagnosis risk.
• Estrogen type matters: CEE (conjugated equine estrogen)can initiate, in some tumors, apoptosis (programmed cell death) , potentially a biological effect distinct from estradiol.

⚰️ What Is the Effect on Breast Cancer Mortality?

WHI assessed not only breast cancer incidence but also death rates (mortality) among participants:

📌 Estrogen was therefore associated with not only fewer cases but also less aggressive breast cancer in the findings.

🧭 Who Should and Should Not Be Considered?

📉 Why Do Observational Studies Produce Different Results?

While observational cohorts show increased breast cancer incidence and mortality with estrogen alone, randomized trials such as WHI report significant reductions in both rates.

These differences may be explained by the high level of evidence from randomized trials and confounding factors in observational research (e.g., the healthy-user effect).

⏳ Treatment Duration and Starting Age: Does This Apply to Every Age Group?

Although studies differ in estrogen formulation, treatment duration, and participant age, their designs show considerable consistency, including WHI.

💊 Comparison with Tamoxifen: What Makes Estrogen Different?

The meta-analysis also compared estrogen’s breast cancer prevention role with tamoxifen:

🧪 Conclusion: Science Is Changing—Should Attitudes Change Too?

✅ For Whom Might Estrogen Alone Be Safe?

This comprehensive meta-analysis provides strong randomized clinical evidence, particularly for postmenopausal women who have had a hysterectomy , that estrogen alone may reduce breast cancer incidence in this population.

For these women, whose quality of life is seriously affected by menopausal symptoms, estrogen is valuable not only for symptom control but also for its potential to reduce breast cancer risk.

I therefore consider this study valuable, but remain cautious about making this routine for all postmenopausal women.