Background Although breast cancer risk is greater in users of estrogen – progestin than estrogen-only formulations of menopausal
hormonal therapy, reports on their effects have been somewhat inconsistent. We investigated whether
the timing of these therapies affected breast cancer incidence.
Methods A total of 1 129 025 postmenopausal UK women provided prospective information on hormonal therapy use and
other factors relevant for breast cancer risk. We used Cox regression to estimate adjusted relative risks(RRs)of
breast cancer in hormonal therapy users vs never users and calculated standardized incidence rates. All statistical
tests were two-sided.