A total of 1,129,025 postmenopausal women in the UK provided prospective information on HRT use and other factors relevant for breast cancer risk.
There were 4.05 million woman-years of follow-up during which 15,759 breast cancers occurred, with 7,107 in current users of HRT. Breast cancer incidence was increased in current HRT users but returned to that of never-users a few years after stopping treatment.
The risk for breast cancer was greater if HRT began before or soon after menopause than if it began after a longer gap. There was little or no increase in risk in women who started estrogen-only HRT 5 years or more after menopause. Risks were greater among users of estrogen-progestogen combinations than in estrogen-only users if hormonal therapy started at around the time of menopause rather than later.
Beral V, Reeves G, Bull D et al. J Natl Cancer Inst 2011; 103: 296-305