Hormone Therapy and Breast Cancer Risk in Young Women

Strategic Insights -

The National Institutes of Health (NIH) conducted a comprehensive study on the impact of hormone therapy on breast cancer risk in women under 55. Researchers analyzed data from over 459,000 women across North America, Europe, Asia, and Australia, revealing that hormone therapy can significantly alter breast cancer risk. Specifically, women treated with unopposed estrogen hormone therapy (E-HT) were less likely to develop breast cancer than non-users, while those treated with estrogen plus progestin hormone therapy (EP-HT) were more likely to develop breast cancer.

Key Takeaways:

  • The study found that women treated with unopposed estrogen hormone therapy (E-HT) had a 14% reduction in breast cancer incidence compared to non-users, with a more pronounced protective effect in women who started E-HT at younger ages or used it longer.
  • Women using EP-HT experienced a 10% higher rate of breast cancer compared to non-users, with an 18% higher rate seen among women using EP-HT for more than two years.
  • The association between EP-HT and breast cancer was particularly elevated among women who had not undergone hysterectomy or oophorectomy.
  • The study suggests that the cumulative risk of breast cancer before age 55 could be about 4.5% for women using EP-HT, compared to a 4.1% risk for non-users and a 3.6% risk for those using E-HT.
  • The findings highlight the importance of considering gynecological surgery status when evaluating the risks of starting hormone therapy.
  • The study is consistent with previous large studies documenting similar associations between hormone therapy and breast cancer risk among older and postmenopausal women, and extends those findings to younger women.

Statistics:

  • 459,000 women under 55 years old were included in the analysis across North America, Europe, Asia, and Australia
  • Women using E-HT had a 14% reduction in breast cancer incidence compared to non-users
  • Women using EP-HT experienced a 10% higher rate of breast cancer compared to non-users
  • Women using EP-HT for more than two years had an 18% higher rate of breast cancer compared to non-users
  • The cumulative risk of breast cancer before age 55 could be about 4.5% for women using EP-HT, compared to a 4.1% risk for non-users and a 3.6% risk for those using E-HT

Sources:

  • O'Brien KM, et al. 2025. Hormone therapy use and young-onset breast cancer: a pooled analysis of prospective cohorts included in the Premenopausal Breast Cancer Collaborative Group. Lancet Oncol 26: 911--23.
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