Estrogen-only hormone therapy does not raise the risk of breast cancer for menopausal women, according to research conducted by the Women's Health Initiative (WHI), an endeavor of the National Institutes of Health. The study results were published in the Journal of the American Medical Association.
The WHI, launched in 1991, involved a set of clinical trials and an observational study, which together included 161,808 generally healthy postmenopausal women. The clinical trials were designed to test the effects of postmenopausal hormone therapy, diet modification, and calcium and vitamin D supplements on heart disease, fractures, and breast and colorectal cancer.
The hormone trial had two studies: an estrogen-plus-progestin study of women with a uterus and an estrogen-alone study of women without a uterus. (Women with a uterus were given progestin in combination with estrogen, a practice known to prevent endometrial cancer.) In both hormone therapy studies, women were randomly assigned to either the hormone medication being studied or to placebo. Researchers observed that in the women treated only with estrogen, there was no significant increased risk of developing cancer.