Although oestrogen levels in oral contraceptives have decreased markedly over the years, with pills in the 1960s typically containing more than double the oestrogen dose of pills in the 1980s, the study found 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.
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“That means women who are taking the pill in their 20s and 30s are protected right through to the age when they are at greatest risk of developing endometrium cancer, which is 55 or older”, she said.
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.
Use of the Pill is already known to offer a degree of protection against both womb and ovarian cancers.
While modern-day birth control contains less hormones than in previous decades, the study suggests that the lower dosage is still sufficient enough to reduce the risk of uterine cancer.
Studies have also linked use of the pill to a slight increase in breast cancer risk.
Researchers pooled data on 27,276 women with the disease and nearly 116,000 who were cancer-free from 36 studies in North America, Europe, Asia, Australia and South Africa.
But the researchers stopped short of suggesting that women start taking oral contraceptives as an anti-cancer measure.
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.
The study also found the risk reduction did not vary substantially by a woman’s reproductive history, amount of body fat, alcohol use, tobacco use or ethnicity. But this re-evaluation of virtually all related epidemiological evidence looking at womb cancer, funded by Cancer Research UK and the Medical Research Council, shines further light on its effect.
In a commentary in the journal on the paper, Nicolas Wentzensen and Amy Berrington de Gonzalez, from the National Cancer Institute in the US, say that the important question now is whether this allows women to better balance the benefits and the potential harms of taking the pill – and also to work out whether the pill might be useful in some circumstances specifically to prevent cancer.
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As soon as women stop taking the pill, the risk of thrombosis and stroke go away, Beral said.
