The contraceptive pill has prevented some 200,000 cases of ovarian cancer and 100,000 deaths from this disease since its introduction nearly half a century ago, according to a study published in next Saturday's Lancet medical journal.
Over the next decade, around 30,000 extra cases of ovarian cancer are likely to be prevented each year because of the pill, it adds.
The figure are extrapolated from an overview of 45 studies in 21 countries involving 23,000 women diagnosed with ovarian cancer and 87,000 who were otherwise healthy.
Women who had been using oral contraceptives were far less likely to have this form of cancer than counterparts who had not been using the pill, the review found.
And the longer a woman had been on the pill, the more the risk diminished.
Ten years on the pill reduced the risk of ovarian cancer before the age of 75 by a third, and the risk of death by 30 percent.
The benefit was still perceptible, if somewhat reduced, more than 30 years after the woman stopped taking the pill.
The paper, based on long-term research, sheds light on the long-term protective advantage of oral contraceptives when it comes to ovarian cancer.
Other research, though, has found a statistically significant increased risk of cancer of the breast, cervix or central nervous system among users of the pill.
Around 120 million women around the world used the pill in 2002, 80 million of them in developing countries, according to figures cited by The Lancet paper.
It is authored by the Collaborative Group on Epidemiological Studies of Ovarian Cancer, sponsored by the British charity Cancer Research UK.
In the 1960s, doses of estrogen in the pill were typically double those of the 1980s, when the hormone formulation was slimmed down.
Even so, there was no apparent change in the relative risk of ovarian cancer among women who used the pill in the 1960s, 1970s and 1980s.
In an editorial, The Lancet called for the pill to be made available over the counter, rather than restricted by a doctor's prescription, given that, in its view, the benefits for cancer prevention and reproductive health so outweighed the risks.
"We believe the case is now convincing," the British journal said.