Study Links Hormone Therapy to Breast Cancer Risk in Women Under 55 - Video

A new large-scale study published in Lancet Oncology has found that two commonly used hormone therapies may affect breast cancer risk in women under the age of 55. Scientists at the National Institutes of Health (NIH) revealed that unopposed estrogen hormone therapy (E-HT) is linked to a reduced risk of breast cancer, while estrogen plus progestin hormone therapy (EP-HT) appears to increase the risk.

E-HT is generally recommended only for women who have had a hysterectomy due to its known association with uterine cancer. EP-HT, on the other hand, is often prescribed to women with an intact uterus to offset the risks of estrogen alone.

The researchers conducted a comprehensive analysis involving over 459,000 women under 55 from North America, Europe, Asia, and Australia. Their results showed that women who used E-HT had a 14% lower incidence of breast cancer compared to non-users, with the protective effect more pronounced in those who started at younger ages or used it longer. Conversely, EP-HT users faced a 10% higher rate of breast cancer, with the risk climbing to 18% for those who used it for more than two years.

According to the study, the estimated cumulative risk of developing breast cancer before age 55 was about 3.6% for E-HT users, 4.1% for non-users, and 4.5% for EP-HT users. Importantly, the increased risk associated with EP-HT was especially evident in women who had not undergone hysterectomy or oophorectomy.

“Hormone therapy can greatly improve the quality of life for women experiencing severe menopausal symptoms or those who have had surgeries that affect their hormone levels,” said lead author Katie O’Brien, Ph.D., of NIH’s National Institute of Environmental Health Sciences (NIEHS). Our study provides greater understanding of the risks associated with different types of hormone therapy, which we hope will help patients and their doctors develop more informed treatment plans.”

The study supports earlier findings on hormone therapy and breast cancer in older women and extends those insights to younger women, offering critical evidence to guide clinical decision-making.

Reference: O’Brien, K., et al. “Hormone therapy use and young-onset breast cancer: a pooled analysis of prospective cohorts included in the Premenopausal Breast Cancer Collaborative Group.” Lancet Oncol 2025; 26: 911–23.