Two types of hormone therapy may alter the risk of breast cancer in women under 55.

Scientists at the U.S. National Institutes of Health (NIH) have found that two common types of hormone therapy may alter the risk of breast cancer in women younger than 55. Researchers found that women treated with unopposed estrogen-containing hormone therapy (E-HT) were less likely to develop the disease than those who did not use it. They also found that women treated with estrogen-progestin hormone therapy (EP-HT) were more likely to develop breast cancer than women who did not use it . Together, these findings could help guide clinical recommendations for the use of hormone therapy in younger women.

The two hormone therapies analyzed in the study are often used to control symptoms associated with menopause or after removal of the uterus or one or both ovaries . Unopposed estrogen therapy is only recommended for women who have had their uterus removed because of its known association with increased risk of uterine cancer.

"Hormone therapy can significantly improve the quality of life for women experiencing severe menopausal symptoms or who have undergone surgery that affects their hormone levels," said Katie O'Brien , lead author of the study.

Researchers conducted a large-scale analysis that included data from more than 459,000 women under the age of 55 in North America, Europe, Asia, and Australia . Women who used hormone replacement therapy (HRT) had a 14% reduction in breast cancer incidence compared to those who never used it. Notably, this protective effect was more pronounced in women who started HRT at younger ages or who used it for longer periods of time. In contrast, women who used HRT experienced a 10% higher rate of breast cancer compared to those who did not use it, with the rate being 18% higher in women who used it for more than two years compared to those who never used it.

"These findings underscore the need for personalized medical advice when considering hormone therapy," says Dale Sandler. " Women and their healthcare providers should weigh the benefits of symptom relief against the potential risks associated with hormone therapy, especially progestogen-containing hormone therapy. For women with an intact uterus and ovaries, the increased risk of breast cancer with progestogen-containing hormone therapy should prompt careful consideration."

This study is consistent with previous large-scale studies that documented similar associations between hormone therapy and breast cancer risk in older, postmenopausal women. This new study extends these findings to younger women.