Menopausal Hormone Therapy and Breast Cancer

Associations of estrogen-alone and estrogen plus progestin with breast cancer incidence and related mortality are reviewed from observational studies (The Collaborative Group on Hormonal Factors in Breast Cancer and The Million Women Study, 2019) and the Women's Health Initiative's (2020) two randomized trials evaluating conjugated equine estrogen alone, for women with prior hysterectomy or with medroxyprogesterone acetate. Findings are generally concordant for estrogen plus progestin use with both observational and randomized studies reporting higher breast cancer incidence. Findings are discordant for estrogen-alone use where, in the WHI randomized trial, a lower incidence and lower breast cancer mortality was seen. In contrast, in the observational studies, estrogen-alone use was associated with higher breast cancer incidence and higher breast cancer mortality. Although these discordant findings are difficult to fully reconcile, we conclude with a discussion of public health implications of the available evidence on menopausal hormone therapy and breast cancer.