NIH Study Reveals Hormone Therapy's Impact on Breast Cancer Risk in Younger Women

In a groundbreaking study published in The Lancet Oncology on July 1, 2025, researchers from the National Institutes of Health (NIH) have uncovered significant findings linking hormone therapy to breast cancer risk in women under the age of 55. The analysis, which involved data from over 459,000 women across North America, Europe, Asia, and Australia, examined the effects of two common forms of hormone therapy: unopposed estrogen hormone therapy (E-HT) and estrogen plus progestin hormone therapy (EP-HT).

Dr. Katie O’Brien, lead author and researcher at the NIH's National Institute of Environmental Health Sciences (NIEHS), indicated that women who used E-HT experienced a 14% lower incidence of breast cancer compared to non-users. This protective effect was particularly notable among those who began treatment at a younger age or used it for extended periods. Conversely, women undergoing EP-HT demonstrated a 10% higher breast cancer incidence compared to those who did not use hormone therapy, with the risk escalating to an 18% increase for those on EP-HT for over two years.

"These findings underscore the need for personalized medical advice when considering hormone therapy," Dr. Dale Sandler, senior author and NIEHS scientist, emphasized. "Women and their healthcare providers should weigh the benefits of symptom relief against the potential risks associated with hormone therapy, especially EP-HT."

The study suggests that the cumulative risk of breast cancer before the age of 55 for EP-HT users could be around 4.5%, in contrast to 4.1% for non-users and 3.6% for E-HT users. Notably, the association between EP-HT and increased breast cancer risk was most pronounced in women who had not undergone hysterectomy or oophorectomy, highlighting the importance of surgical history in evaluating hormone therapy risks.

This research builds on previous studies that indicated similar risks for older and postmenopausal women, extending these findings to a younger demographic—essential for guiding clinical recommendations for hormone therapy use. The implications are particularly significant given that hormone therapy is frequently prescribed to manage menopausal symptoms and post-surgical conditions.

Experts in the field stress the importance of the study's findings in informing treatment decisions. Dr. Sarah Johnson, Professor of Oncology at Johns Hopkins University, remarked, "The results of this analysis provide a crucial framework for understanding the nuanced risks associated with various hormone therapies, allowing for better-informed choices by patients and their physicians."

Researcher insights indicate that while hormone therapy can significantly alleviate menopausal symptoms, the potential risks, particularly with EP-HT, warrant comprehensive discussions between women and their healthcare providers. The findings are expected to contribute to a growing body of literature aimed at optimizing hormone therapy protocols, ensuring women receive care tailored to their individual health profiles.

As the conversation around women’s health continues to evolve, the NIH study serves as a pivotal reference point for ongoing discussions regarding hormone therapy, breast cancer risk, and the importance of personalized healthcare approaches in women’s medicine. The growing body of evidence highlights the necessity of continuous research in this area to better understand the long-term effects of hormone therapy across diverse populations, ultimately leading to improved health outcomes for women worldwide.