Oral contraceptives have prevented about 200,000 cases of endometrial cancer in the last decade

Researchers from the Collaborative Group on Epidemiological Studies on Endometrial Cancer estimate that in the past 50 years (1965-2014) about 400000 cases of endometrial cancer have been prevented by oral contraceptive use in high-income countries, including about 200000 in the last decade (2005-2014).

"The strong protective effect of oral contraceptives against endometrial cancer -- which persists for decades after stopping the pill -- means that women who use it when they are in their 20s or even younger continue to benefit into their 50s and older, when cancer becomes more common," explains study author Professor Valerie Beral, from the University of Oxford in the UK.

She added "Previous research has shown that the pill also protects against ovarian cancer. People used to worry that the pill might cause cancer, but in the long term the pill reduces the risk of getting cancer."

The researchers pooled data on 27276 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.

The findings reveal that every 5 years of oral contraceptive use 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.

Although estrogen doses in oral contraceptives have decreased appreciably over the years, with pills in the 1960s typically containing more than double the estrogen dose of pills in the 1980s, the reduction in endometrial cancer risk was at least as great for women who used the pill during the 1980s as for those who used it in earlier decades. These results suggest that the amount of hormones in the lower-dose pills is still sufficient to reduce the incidence of endometrial cancer, say the authors.

The proportional risk reduction did not vary substantially by women's reproductive history, adiposity (amount of body fat), alcohol use, tobacco use, or ethnicity.

According to study author Dr Naomi Allen, also from the University of Oxford, UK, "The existing evidence suggests that medium-to-long-term use of oral contraceptives (ie, for 5 years or longer) results in substantially reduced risk of endometrial cancer. Over the past 50 years (1965-2014), we estimate that about 400000 endometrial cancers have been prevented in women before the age of 75 years in high-income countries through the use of oral contraceptives, with about 200,000 prevented during the last decade (2005-14)."

This study was funded by the Medical Research Council and Cancer Research UK.