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The U.K. Royal College of General Practitioners' Oral Contraceptive Study, begun in 1968, is the longest-running study of the health effects of combined oral contraceptives (OCs). As of 2012, mean age of participants was 70, mean duration of OC use was 3.7 years, and median follow-up was 41 years in a cohort that included 4661 ever users with at least one cancer diagnosed during 885,000 woman-years and 2341 never users with at least one cancer during 386,000 woman-years.
In ever users compared with never users, risks for any cancer (incidence rate ratio, 0.96) or breast cancer (IRR, 1.04) were essentially identical whereas risks for colorectal (IRR, 0.81), endometrial (IRR, 0.66), or ovarian cancer (IRR, 0.67) were reduced (P<0.05 for all co…