Researchers from the Collaborative Group on Epidemiological Studies on Endometrial Cancer estimate that in the past 50 years (1965 to 2014), about 400,000 cases of endometrial cancer have been prevented by oral contraceptive use in high- income countries, including about 200,000 in the last decade (2005 to 2014).
The findings reveal that every five years of taking the pill reduces the risk of endometrial cancer by about a quarter. In high-income countries, 10 years of oral contraceptive use reduces the risk of developing endometrial cancer before age 75 from 2.3 to 1.3 cases per 100 users.
Indeed a protective effect persists for at least 30 years after use ceases, and does not seem to depend much on the dose of oestrogen in the pill (doses were far higher in the 1960s) or on personal characteristics such as reproductive history, body fat, menopausal status, use of HRT, smoking status, or alcohol intake.
The findings, published in The Lancet Oncology journal, mean that women who use the pill in their 20s or even younger continue to benefit into their 50s and older, when cancer becomes more common, according to study author Prof Valerie Beral from the University of Oxford in Britain.
This meant the public health effect was most apparent many years after use has stopped.
Dr Shirley McQuade, medical director of the Dublin Well Woman Centre, said there were other beneficial effects to taking the pill “that rarely get talked about”, including that it reduces the risk of ovarian cysts, as well as helping regulate menstrual periods in girls and women with polycystic ovarian syndrome. Polycystic ovarian syndrome is linked to hormonal imbalance and can lead to difficulties conceiving.
The pill was also one of the “first line treatments” for endometriosis, Dr McQuade said, which affects about 10% of the female population, where tissue that normally grows inside the uterus grows outside it instead.
Prof Beral said: “People used to worry that the pill might cause cancer, but in the long-term the pill reduces the risk of getting cancer”.
She added that previous research had shown the pill also protects against ovarian cancer.
In conducting their study, researchers pooled data on 27,276 women with endometrial cancer in 36 studies from North America, Europe, Asia, Australia, and South Africa — virtually all the epidemiological evidence ever collected on the effect of oral contraceptives.
This study was funded by the Medical Research Council and Cancer Research UK.