THE USE OF ESTROGENS AND PROGESTINS AND THE RISK OF BREAST-CANCER IN POSTMENOPAUSAL WOMEN

Background. The effect of adding progestins to estrogen therapy on the risk of breast cancer in postmenopausal women is controversial. Metho ds. To quantify the relation between the use of hormones and the risk of breast cancer in postmenopausal women, we extended our follow-up of the participants in the Nurses' Health Study to 1992. The women were asked to complete questionnaires every two years to update information on their menopausal status, use of estrogen and progestin preparation s, and any diagnosis of breast cancer. During 725,550 person-years of follow-up, we documented 1935 cases of newly diagnosed invasive breast cancer. Results. The risk of breast cancer was significantly increased among women who were currently using estrogen alone (relative risk, 1 .32; 95 percent confidence interval, 1.14 to 1.54) or estrogen plus pr ogestin (relative risk, 1.41; 95 percent confidence interval, 1.15 to 1.74), as compared with postmenopausal women who had never used hormon es. Women currently taking hormones who had used such therapy for 5 to 9 years had an adjusted relative risk of breast cancer of 1.46 (95 pe rcent confidence interval, 1.22 to 1.74), as did those currently using hormones who had done so for a total of 10 or more years (relative ri sk, 1.46; 95 percent confidence interval, 1.20 to 1.76). The increased risk of breast cancer associated with five or more years of postmenop ausal hormone therapy was greater among older women (relative risk for women 60 to 64 years old, 1.71; 95 percent confidence interval, 1.34 to 2.18). The relative risk of death due to breast cancer was 1.45 (95 percent confidence interval, 1.01 to 2.09) among women who had taken estrogen for five or more years. Conclusions. The addition of progesti ns to estrogen therapy does not reduce the risk of breast cancer among postmenopausal women, The substantial increase in the risk of breast cancer among older women who take hormones suggests that the trade-off s between risks and benefits should be carefully assessed.