An epidemiological overview of the relationship between hormone replacement therapy and breast cancer.

Large-scale randomized clinical trials and observational studies have consistently found that use of hormone replacement therapy (HRT) increases the risk of breast cancer. More recently, ecological studies have shown correlations between dramatic reductions in use of HRT in many countries, and declines in the rates of breast cancer in older women. Meta-analyses of data from the trials and observational studies show that the increase in breast cancer risk is greater for combined estrogen-progestin therapies compared with estrogen alone; that for both types of preparation, breast cancer risk increases with duration of use; and that the risks decrease relatively quickly after cessation of use. For both estrogen-only and combined therapies, the risk of breast cancer is higher if therapy is initiated close to the time of the menopause, relative to the risks in women starting HRT later. Most drug regulatory authorities currently recommend that HRT be prescribed only to fully informed women who have moderate-to-severe menopausal symptoms, for the shortest duration possible; and it is recommended that the need for therapy be reviewed at least every 6-12 months.