Oral contraceptive use has been suggested to influence the risk of breast, ovarian, and endometrial cancer. The purpose of this study is to clarify the time-dependent effects between long-term oral contraceptive use and cancer risk. We performed an observational study in 256,661 women from UK Biobank, born between 1939 and 1970. Information on cancer diagnoses were collected from self-reported data and from national registers until March 2019. Cumulative risk of cancer over the timespan of the study, as measured by the odds ratio (OR), and instantaneous risk, as measured by the hazard ratio (HR), were assessed using Logistic and Cox regression analyses, respectively. The odds were lower among ever users, compared with never users, for ovarian cancer: OR=0.72 (95% CI: 0.65-0.81) and endometrial cancer: OR=0.68 (95% CI: 0.62-0.75), an association that was stronger with longer use (P<0.001). Increased odds were seen for breast cancer in women when limiting the follow-up to 55 y ears of age: OR=1.10 (95% CI: 1.03-1.17), but not for the full timespan. We only found a higher HR for breast cancer in former users immediately (≤2 years) after discontinued oral contraceptive use (HR=1.55, 95% CI: 1.06-2.28), whereas the protective association for ovarian and endometrial cancer remained significant up to 35 years after last use of oral contraceptives. Given the body of evidence presented in our study, we argue that oral contraceptives can dramatically reduce women's risk of ovarian and endometrial cancer, whereas their effect on lifetime risk of breast cancer is limited.
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