Breast cancer and hormone-replacement therapy in the Million Women Study.

Abstract (PubMed)

Current use of hormone-replacement therapy (HRT) increases the incidence of breast cancer. The Million Women Study recruited 1,084,110 UK women aged 50-64 years between 1996 and 2001 and linked them to cancer registry and death records. Among 828,923 postmenopausal women, 9364 developed breast cancer and 637 died from breast cancer over 2.6 million person-years of follow-up. Current HRT users had significantly increased breast cancer incidence compared to never users: combined oestrogen-progestagen preparations were associated with twice the risk (RR 2.00; 95% CI 1.91–2.09), and oestrogen-only preparations with a smaller but significant increase (RR 1.30; 95% CI 1.22–1.38). The increase in incidence was directly related to the duration of use and ceased after stopping HRT. These results confirm that current use of HRT is associated with an increased risk of breast cancer, with the risk greatest for combined oestrogen-progestagen preparations.