Oral contraceptives and breast cancer: what has 20 years of research shown?

This article reviews 20 years of research to determine if oral contraceptive usage increases the risk of breast cancer. Studies using animal models, epidemiology, and human tissues are presented. Results suggest oral contraceptives do not initiate breast cancer, but may act as tumor promoters, stimulating the growth of already transformed cells. The answers are not all in, and the need for continued research is emphasized.