Association of menopausal hormone therapy with long-term breast cancer incidence and mortality

Mixed results from observational studies and randomized clinical trials (RCTs) have created uncertainty about the effect of menopausal hormone therapy on breast cancer, but researchers hoped long-term followup of two RCTs might offer clarity. The Women’s Health Initiative clinical trials welcomed postmenopausal participants, aged 50–79 years, with no breast cancer history and negative screen at baseline. Enrollment ran from 1993 to 1998, with followup continuing through 2017. Collectively, the two trials involved 27,347 patients, including 10,739 who had previously undergone hysterectomy and 16,608 whose uterus was still intact. More than 20 years of followup revealed a significant association between prior randomized use of estrogen and reduced risk of breast cancer and breast cancer mortality compared with placebo. However, prior randomized use of estrogen plus progestin significantly correlated, compared with placebo, to an elevated risk of breast cancer in women with a uterus—although breast cancer mortality was not significantly increased.