Oral contraceptives and breast cancer

Abstract

The incidence of breast cancer among users and non-users of oral contraceptives (OCs) was determined at Group Health Cooperative of Puget Sound, Seattle, Washington, a health care organization which maintains computer files of diagnoses and outpatient drug use. In women 45 years of age or younger, the incidence was nearly identical in users and non-users. In premenopausal women over 45 years of age, there was a positive association between current OC use and breast cancer, the risk ratio estimates and 90% confidence intervals being 4.0 (1.8-9.0) in women 46-50 years of age and 15.5 (5.2-46) in women 51-55 years of age.

PIP: A study on the possible effect of (OCs) oral contraceptives on breast cancer risk was conducted at the Group Health Cooperative of Puget Sound in Seattle, Washington. Exposure and menopausal data were available on women and controls. Among the women aged 31-5, 20% used OCs during the period ending June 30, 1976 as compared with 13% 2 years later. For those women aged 51-5, user prevalence declined from 2% to 0.3%. 76 of 132 women with breast cancer were premenopausal and 4 were under 30 years of age. Estimates of breast cancer rates in the group of current OC users and nonusers were not adjusted for potential confounding by other risk factors; these were later assessed into a multiple logistic function. Those factors were: age, ponderal index, age at menarche, age at 1st pregnancy, history of benign breast disease, education, and race. Current OC use was entered into the risk function in the form of age-use interaction terms. 1 aspect of the case-control comparison which did not correspond to the population-based findings was the apparent protective effect of current OC use in the group ages 31-40. Among those currently using OCs, there is a preponderance of long-term users; no such trend exists among past users. Data from this study indicate that there may be a relation between OC use and breast cancer which is age dependent. Within the age group 31-45 years, the incidence in current users (0.71/1000 women-years) was nearly identical to nonusers (0.65/1000 women-years) however, current OC use was associated with a strong increase in breast cancer risk in premenopausal women 46-55 years. Other factors for which the breast cancer risk ratio may increase with age include pregnancy, and endocrine risk factor. Relationships previously noted which are included in these data include associations between risk and nulliparity, educational level, and slimness (for women who are premenopausal).