At least 100,000 deaths from ovarian cancer have been prevented worldwide by the contraceptive pill over 50 years, research has concluded.
The Oxford University team said the pill's rising popularity meant 30,000 new cases will soon be avoided each year, the Lancet reported.
The findings were based on analysis of 45 previous studies.
Calls for the pill to be available without prescription were strengthened by the study, the Lancet's editor said.
"We strongly endorse more widespread over-the-counter access to a preventive agent that can not only prevent cancers but also demonstrably save the lives of tens of thousands of women"
The link between oral contraceptives and lower rates of ovarian cancer is long-established, but the study is one of the most detailed attempts to work out how effective it is across a woman's lifetime.
Even though the dose of hormones in the 1960s and 1970s pill was roughly double the amount in today's versions, this did not seem to make a difference to the level of protection offered, the researchers said.
And they said protection against cancer can continue decades after a woman has stopped taking the pill.
The relationship between the contraceptive pill and cancer is not all good news - there have been fears about short-term increases in the risk of breast and cervical cancer.
But researcher Sir Richard Peto said that young women did not have to worry about this risk.
"The eventual reduction in ovarian cancer is bigger than any increase in other types of cancer caused by the pill," he said.
Dr Lesley Walker, of Cancer Research UK, said: "All women who have taken the pill or are currently taking it should be reassured by this study."
Easy access
She urged anyone with concerns about it to talk to their GP or family planning clinic.
Julie Bentley, from the FPA (formerly the Family Planning Association) said: "This is great news for women as it is further assurance that the contraceptive pill provides lasting protection against ovarian cancer."
And Lancet editor Richard Horton said that the new evidence was a compelling reason to make it easier to obtain the pill.
"There are few drugs available that confer powerful and long-lasting protection against a highly lethal malignancy after such a short exposure.
"We strongly endorse more widespread over-the-counter access to a preventive agent that can not only prevent cancers but also demonstrably save the lives of tens of thousands of women."
Currently, a woman who wants to take the pill needs a prescription from her GP to do so as it should not be given to women with a history of blood clots and heart and liver disease.
The Medicines and Healthcare Products Regulatory Agency has said there are no plans to make it available over the counter.
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