Oral contraceptives and cancer risk.

Population-based cancer registries in 8 geographic regions across the US were used to identify women 20-54 years of age with newly diagnosed breast ovarian or endometrial cancer. Study controls were women of the same ages without cancer chosen from the same geographic areas by dialing randomly selected telephone numbers. The relative risk of ovarian cancer for women who had used oral contraceptives (OCs) for at least 1 month as compared with women who had never used OCs was 0.6. The longer a woman had used OCs the lower her risk of developing ovarian cancer. The protective effect of OC use persisted more than 10 years after OC use was discontinued. The relative risk of endometrial cancer for women who had used combined OCs containing both an estrogen and a progestin wa 0.5 Women who had used sequential OCs appeared to have an increased risk of endometrial cancer. The protective effects of combined OCs against endometrial cancer appeared to be restricted to women who had used them for 1 year or longer and was concentrated in nulliparous women. For breast cancer women who had used OCs had a relative risk of 9.9 compared with women who had never used OCs. There was no evidence that longterm OC use of more than 10 years or OC use that began 16 or more years earlier shortly after OCs were introduced in the US increased the risk of breast cancer. There was no indication of any increased risk of breast cancer due to OC use for high-risk women.