For every 5 years of birth control pill use, researchers say the risk of endometrial cancer is reduced by around 25%.
"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," says lead study author Prof. Valerie Beral, of the University of Oxford in the UK.
Endometrial cancer is a cancer that begins in the inner lining of the uterus, called the endometrium. The cancer is most common among women over the age of 45, and 3 in every 4 cases are diagnosed among women aged 55 and older.
It is estimated that almost 55,000 women in the US will be diagnosed with endometrial cancers and uterine sarcomas this year, and more than 10,000 women will die from the conditions.
Previous research has associated use of the birth control pill with reduced endometrial cancer incidence rates, but according to Prof. Beral and colleagues, it was unclear whether the pill offers protection against the disease after its use has ceased.
Risk of endometrial cancer reduced by 25% for every 5 years of birth control pill use
To find out, the team collected data from 36 epidemiological studies from Europe, North America, Australia, Asia and South Africa that looked at the effects of the birth control pill. The team's dataset included 27,276 women with endometrial cancer and 115,743 women without the disease.
- The chance of a woman being diagnosed with endometrial cancer in her lifetime is 1 in 37
- Though endometrial cancer is more common in white women, black women are more likely to die from the disease
- Around 600,000 women in the US are survivors of endometrial cancer.
The researchers calculated that the risk of endometrial cancer reduces by around 25% for every 5 years of birth control pill use. In high-income countries, the team estimated that 10 years of using the pill reduces the risk of endometrial cancer from 2.3 cases per 100 users to 1.3 cases.
The reduction in endometrial cancer risk persisted for over 30 years after birth control pill use had ceased, according to the results.
The association between use of the pill and reduction in endometrial cancer did not appear to be influenced by women's reproductive history, body fat, alcohol and tobacco use or ethnicity.
In addition, despite estrogen doses in birth control pills decreasing over the past 50 years, women who used the pill in 1980s - when estrogen doses were less than double what they were 20 years earlier - experienced the same reduction in endometrial cancer as those who used the pill in the 1960s.
"These results show that the amount of estrogen in the lower-dose pills is still sufficient to reduce the incidence of endometrial cancer," note the study authors.
Based on their findings and the patterns of birth control pill use over the past 50 years, the team estimates that between 1965 and 2014, use of the birth control pill has prevented around 400,000 cases of endometrial cancer in developed countries, with around 200,000 of these cases prevented between 2005 and 2014.
The researchers add:
"Since the introduction of oral contraception in the early 1960s, about 400 million women have used it in high-income countries alone, often for prolonged periods during early adulthood.
Medium-to-long-term use of oral contraceptives (eg, for 5 years or longer) results in a substantial proportional reduction in the incidence of endometrial cancer, the magnitude of which is similar to that seen for ovarian cancer."
Some previous studies have indicated that the use of birth control pills may raise cancer risk. In August 2014, for example, Medical News Today reported on a study suggesting recent use of the birth control pill may increase the risk of breast cancer, though this risk may be dependent on the formulation of the pill.