A case-control study of breast cancer in relation to oral contraceptive use in Slovenia.

With the aim to investigate a possible association between oral contraceptive (OC) use and breast cancer occurrence, 534 women aged 24--54 years with newly diagnosed breast cancer and 1989 individually matched hospital controls were interviewed during 1980--1983. The overall risk for ever-users vs. never-users estimated by logistic regression and adjusted for several possible confounding factors was 1.62 (p less than 0.05). The analysis of potential biases indicated that this risk may be overestimated, especially because the controls might not be fully representative of the basic population. The risk was increasing with total duration of OC use, reaching the highest value by more than 7 years of use. As to the latency, the risk was the highest for women starting pill use 4--8 years before diagnosis, thus suggesting that OCs might act as promoters rather than initiators of tumor growth. There was no substantial difference in risk between women starting pill use before 25 years of age and those starting it later. The number of users before first term pregnancy was too small to warrant relative risk estimation. Interaction (significant) was found between OC use and family history of breast cancer; there was no such evidence in other subgroups of women being at baseline breast cancer risk. There were no significant differences in the distribution of cases and controls classified by individual OC formulations used. The increased relative risk for users was concentrated in early stages of breast cancer, most likely owing to detection bias. Considering the indicated biases, the results of the study may not be quite conclusive as to the adverse effect of OCs on the breast, but they call for further investigation of this problem.