A new study suggests that birth control pills may also help shield women from uterine cancer.
Taking birth control pills, even for just a few years, offers significant long-term protection against uterine cancer, also known as endometrial cancer, the British researchers said. And the longer a women takes birth control pills, the greater her reduction in risk for the disease, concluded the team led by Valerie Beral of the University of Oxford.
In fact, they estimated that over the past 50 years, birth control pills have prevented about 400,000 cases of endometrial cancer among women before age 75 in wealthy nations, including about 200,000 between 2005 and 2014 alone.
"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," Beral said in a news release from the journal The Lancet Oncology. The study was published in the journal Aug. 4.
As part of their research, Beral's team analyzed data from 36 studies involving a total of more than 27,000 women with uterine cancer around the world.
While the study couldn't prove cause-and-effect, the results show that every five years of birth control pill use cut the odds of uterine cancer by about one-quarter.
In high-income nations, 10 years of birth control pill use lowered the risk of developing the disease before age 75 from 2.3 to 1.3 cases per 100 users, the study found.
"Previous research has shown that the pill also protects against ovarian cancer," Beral noted. "People used to worry that the pill might cause cancer, but in the long term the pill reduces the risk of getting cancer."
Levels of the hormone estrogen in birth control pills have also decreased substantially over the years, the authors said. Pills in the 1960s typically contained more than double the amount of estrogen than pills in the 1980s did.
Even so, the reduction in uterine cancer risk was at least as large for women who used birth control pills in the 1980s as for those who used them in the 1960s, the research showed.
This suggests that estrogen amounts in lower-dose pills are still sufficient to reduce the risk of uterine cancers, Beral's team said.
They also found that a woman's reproductive history, amount of body fat, alcohol and tobacco use, or ethnicity had little effect on the amount of protection birth control pills provide against uterine cancer.
One expert in the United States said the findings should come as good news for women.
"One of the most impressive aspects of the studies showed that the reduction in risk persists long after the patient ceases use of the pill, even up to 30 years later," said Dr. Jennifer Wu, an obstetrician-gynecologist at Lenox Hill Hospital in New York City.
The study was funded by the Medical Research Council and Cancer Research U.K.