Oral-contraceptive use and the risk of breast cancer. The Cancer and Steroid Hormone Study of the Centers for Disease Control and the National Institute of Child Health and Human Development.

Data from the Cancer and Steroid Hormone Study of the Centers for Disease Control (CDC) were used to analyze the effect of oral contraceptives (OCs) on the risk of breast cancer in women 20-54 years of age. Cases included 4711 women with newly diagnosed primary breast cancer; 4676 controls were selected through random-digit dialing in the geographic areas of the cases. A higher proportion of cases than controls were nulliparous older at 1st term pregnancy had a history of benign breast disease were premenopausal were younger at menarche and had fewer live births. Overall the relative risk of breast cancer was 1.0 in women who had used OCs compared to never-users; that is no difference between the two groups. Even when OC use was categorized according to both duration of use and time since last use no pattern of association between OC use and breast cancer was apparent. In addition use of specific formulations of OCs and the type of estrogen or progestin in combination OCs were not significant factors. The relative risk of breast cancer was 1.0 both among women who used OCs containing only mestranol and among those who used formulations containing only ethinyl estradiol as the estrogen component. The relative risk of breast cancer among women who had used only combination OCs ranged from0.6 (10 mg norethindrone/60 mcg mestranol) to 1.6 (1 mg norethindrone acetate/50 mcg ethinyl estradiol) but no single preparation was associated with a significantly increased risk. Overall these results provide further support for the contention that OC use does not increase the risk of breast cancer even when the contraceptives have been used for 15 years or longer.