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To investigate cancer risk among users of oral contraceptives (OCs), researchers analyzed 36 years of data from the U.K. National Health Service (NHS) and participating British general practitioners on OC use and incidence of various cancers. The study included 745,000 and 339,000 woman-years of observation for OC ever-users and never-users, respectively (NHS dataset), and 331,000 and 224,000 woman-years for ever-users and never-users, respectively (GP dataset).
The NHS data showed that ever-users of OCs had statistically significant relative risk reductions of 12% for any type of cancer and 30% for gynecologic cancers compared with never-users. The GP data demonstrated a 3% generalized cancer risk reduction (not statistically significant) a…