It's long been known that the use of oral contraceptives by women can help prevent the onset of ovarian cancer, but a new study by some of the same researchers shows that the protective effects of the pill are even more profound than what scientists had previously believed.

Researchers from Oxford University in the UK have found that oral contraceptives can help reduce the risk of womb cancer (aka endometrial cancer) in women decades after they've taken the medication, with estimates that as many as 400,000 cases of the disease have been prevented by women taking the pill when they were young.

"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," said Valerie Beral, lead author of the study, in a statement to the press.

The researchers analysed a huge amount of information collected from previous research on the pill, collecting 36 studies from around the world that contained data on 27,276 women with womb cancer. Beral and her team found that the protective effects of the pill appear to be cumulative, with every five years of oral contraceptive use when young corresponding to a 25 per cent reduction in the likelihood of womb cancer later in life.

This reduction in risk adds up to huge societal health benefits over time. The results of the study, published in The Lancet, contain estimates that 400,000 incidences of cancer have been prevented in the past 50 years, with 200,000 of those cases having been prevented in the past decade alone. In high-income countries, researchers say 10 years' use of the pill is estimated to reduce the absolute risk of womb cancer arising in women younger than 75 from two out of three to just one out of three. The researchers note that the data suggests the reduction in risk of cancer is more pronounced for carcinomas than sarcomas.

All up, it's an astonishing and unforeseen side effect conferred by what has at times been an extremely controversial medication around the world. Amazing to think how many women in society – our mothers, aunts and grandmothers – might have already benefited from this unintended consequence of the pill. An unexpected advantage they could never have dreamed of when as young women they decided to take a drug purely designed to prevent pregnancy.

The research will also inevitably have serious implications for the young women of today, who may well consider taking the pill for its long-term health benefits in addition to its contraceptive purpose.

"People used to worry that the pill might cause cancer," said Beral, "but in the long term the pill reduces the risk of getting cancer."

A commentary on the research by scientists from the US National Cancer Institute says the new statistically based findings should be taken into account in the wider context of what we know about the risks and benefits of the pill.

The commentary's authors, Nicolas Wentzensen and Amy Berrington de Gonzalez, conclude, "Even if the biological mechanisms remain elusive and the existing evidence falls short of wider recommendations for [prescribing the pill to prevent cancer], women need to be more aware of the unintended benefits and the risks of oral contraceptives, so that they can make informed decisions."