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

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.