Results
During 4.05 million woman-years of follow-up, 15 759 incident breast cancers occurred, with 7107 in current users of hormonal therapy. Breast cancer incidence was increased in current users of hormonal therapy, returning to that of never users a few years after use had ceased. The relative risks for breast cancer in current users were greater if hormonaltherapy was begun before or soon after menopause than after a longer gap ( P heterogeneity < .001, for both estrogen-only and estrogen-progestin formulations). Among current users of estrogen-only formulations , there was little or no increase in risk if use began 5 years or more after menopause ( RR = 1.05, 95% confidence interval [CI] = 0.89 to 1.24), but risk was statistically significantly increased if use began before or less than 5 years after menopause (RR =
1.43, 95% CI = 1.35 to 1.51). A similar pattern was observed among current users of estrogen – progestin formulations(RR = 1.53, 95% CI = 1.38 to 1.70, and RR = 2.04, 95% CI = 1.95 to 2.14, respectively). At 50 – 59 years of age , annual standardized incidence rates for breast cancer were 0.30 % (95% CI = 0.29% to 0.31%) among never users of hormone therapy and 0.43% (95% CI = 0.42% to 0.45%) and 0.61% (95% CI = 0.59% to 0.64%), respectively, among current users
of estrogen-only and estrogen – progestin formulations who began use less than 5 years after menopause.