In addition to their reproductive benefits, oral contraceptives (OCs) provide long-term protection against endometrial cancer, according to a new meta-analysis in The Lancet of data from 36 epidemiological studies. The findings suggest that, over the past 50 years in developed countries, OCs have prevented 400,000 cases of the disease in women younger than age 75.
Researchers from the Collaborative Group on Epidemiological Studies on Endometrial Cancer assessed individual information from the 36 studies on 27,276 women who had endometrial cancer and 115,743 women who didn’t have the cancer and served as controls. Logistic regression was used to estimate the relative risks of endometrial cancer associated with OC use. Among the cases, the median age was 63 years and the median year of cancer diagnosis was 2001.
Overall, 9459 of the cases and 45,625 of the controls had any history of using OCs, with a median duration of 3.0 years for the cases and 4.4 years for the controls. The research showed that the longer a woman used OCs, the greater the reduction in her risk of endometrial cancer. Every 5 years of OC use was linked to a risk ratio (RR) of 0.76 (95% CI 0.73-0.78; P < 0.0001). The risk reduction persisted for more than 3 decades after OC use ceased and the RRs did not decline during the 1960s, 1970s, and 1980s, even as the estrogen doses in OCs was reduced.
Risk reduction did differ by tumor type. It was stronger in sarcomas (RR 0.69; 95% CI 0.66 – 0.71) than it was in sarcomas (RR 0.83; 95% CI 0.67 – 1.04; case-case comparison P = 0.02). In high-income countries, a history of 10 years of OC use was estimated to reduce the absolute risk of endometrial cancer arising before age 75 years from 2.3 to 1.3 per 100 women.
Of the roughly 400,000 cases of endometrial cancer in women before age 75 that the investigators believe have been prevented by OC use from 1965 to 2014, half would have occurred in the past decade alone.