Combined oral contraceptive use and epithelial ovarian cancer risk - Time-related effects

Background: Although the protective effect of oral contraceptives (OCs) use against epithelial ovarian cancer is well-established, there remain gaps in our understanding of the contributions of time-related characteristics of OC use to risk. Methods: This population-based case-control study, carried out in Hawaii and Los Angeles 1993-2006, included 813 cases of epithelial ovarian cancer and 992 controls. Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated using unconditional logistic regression. Results: Epithelial ovarian cancer risk was reduced 5 or more years after initiation of OC use (OR = 0. 18; CI = 0.08 - 0.3 9). Each year of use provided a 5% reduction (CI = 2%-8%) in risk. A positive gradient in risk with time since first OC use was independent of duration of OC use. The inverse association of OCs with risk was attenuated decades after last use, but was not affected by age at first or last use. OC use for <1 year was associated with decreased ovarian cancer risk (OR = 0.45; CI = 0.26-0.79) only among recent users (<= 20 years from diagnosis/interview). Women who used OCs for a year or more were protected for at least 3 decades after they stopped use. Conclusions: Reduction in epithelial ovarian cancer risk associated with OC use became apparent after a short latency period and short duration of use, and was long-lasting. Time since first use and time since last use seem to modify the association of OCs with ovarian cancer risk independently of duration of use.