An estimated 200,000 cases of womb cancer have been prevented in the last decade by women taking the Pill, say scientists.
Oral contraceptives are known to reduce the risk of endometrial cancer, which affects the lining of the womb, but the extent of the protection has been unclear until now.
Researchers pooled data on 27,276 women with the disease and almost 116,000 who were cancer-free from 36 studies in North America, Europe, Asia, Australia and South Africa.
They found that every five years on the Pill cut the chances of developing womb cancer by a quarter in high-income countries.
Ten years of oral contraceptive use lowered the incidence of women diagnosed with the disease before the age of 75 from 2.3 per 100 to 1.3.
The results, published in The Lancet Oncology journal, suggest that between 2005 and 2014 Pill use prevented some 200,000 cases of endometrial cancer.
Looking further back over 50 years, to when oral contraceptives were introduced in the 1960s, suggested a total of 400,000 cases avoided.
Professor Valerie Beral, from the University of Oxford, a leading member of the international team, said: “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.
“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 protective element in the Pill that cuts the risk of womb cancer is the hormone oestrogen. But even though oral contraceptives in the 1960s contained more than double the oestrogen dose used in the 1980s, the level of protection against womb cancer did not differ between the two decades.
This indicated that the hormone content of the modern low-dose Pill was enough to reduce the likelihood of developing endometrial cancer, said the scientists.
Co-author Dr Naomi Allen, also from the University of Oxford, said: “The existing evidence suggests that medium to long-term use of oral contraceptives (ie, for five years or longer) results in substantially reduced risk of endometrial cancer.”
The study was funded by the Medical Research Council and Cancer Research UK.
While the Pill protects against womb cancer, research suggests it slightly increases the risk of breast cancer. However the risk starts to drop when a woman stops taking oral contraceptives.
Cancer Research UK advises women to weigh up the risks, especially those with a family history of breast cancer.