From: What Your Doctor May Not Tell You About Breast Cancer: How Hormone Balance Can Help Save Your Life
by John Lee, M.D.
Warner Books, 2003

Risk Factors for Breast Cancer: Oral Contraceptives Given to Teenagers

Use of oral contraceptives by teens is now an established risk factor for breast cancer. The younger the girl, the higher her risk of breast cancer. In general, girls under the age of 18 who use oral contraceptives triple their lifetime risk of breast cancer. Again, this is most likely due to the progestins (synthetic progesterone) in the birth control pills (or shots) blocking the beneficial actions of real progesterone, and also blocking ovulation and thus the production of a woman's own hormones.

In women older than 20, taking oral contraceptives long term, and for ten years afterward, confers a slightly higher risk of breast cancer. In women in their thirties, however, birth control pills that are testosterone-derived, such as norgestrel, appear to confer some protection against breast cancer. This may be because testosterone is a major antagonist of estrogen, combined with the fact that birth control pills create a lower overall hormonal milieu.