The international team of experts says widespread use of the pill in developed countries is likely to have saved hundreds of thousands of lives since the 1960s, and women should feel confident that the benefits of the drug outweigh its risks when it comes to cancer.

The study of more than 27,000 women found that for every five years a woman was on the pill, her risk of cancer of the endometrium, which lines the uterus, decreased by about 25 per cent.

The benefits increase the longer a woman is on the pill, and occur independently of a range of cancer risk factors including weight, use of menopause hormone therapy, smoking and whether or not she has had children.

Associate Professor Karen Canfell, the director of cancer research at Cancer Council NSW and a member of the international collaborative group that produced the research, said it was particularly reassuring because it showed the benefits lasted even after a woman stopped taking the pill.

"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. "It's a really important result, and it's a really reassuring result."

Endometrium cancer is the most common form of uterine cancer. It is diagnosed in about 2500 Australian women each year and kills more than 400.

Research into the effect hormonal contraceptives had on cancer had been mixed, and it appeared that women on the pill still had an increased risk of breast cancer and cervical cancer while they were taking it, Associate Professor Canfell said.

"But the good news is, it is a relatively modest and transient risk, so it goes back to normal within 10 years," she said. "I think if you look at a woman's overall lifetime risk of developing any cancer, the benefit in terms of ovarian and endometrium cancer outweighs the transient risk in terms of breast and cervical cancer."

She said it was not known why oral contraceptive use would lower the risk of some cancers but raise the risk of others.

The research team said it appeared decreasing amounts of hormones used in contraceptive pills in the 1980s compared with the 1960s had not decreased the benefit for women who used them.

"Since the introduction of oral contraception in the early 1960s, about 400 million women have used it in high-income countries alone, often for prolonged periods during early adulthood," the researchers wrote in an article published in The Lancet Oncology on Wednesday morning.

"The present results . . . suggest that in high-income countries, oral contraceptives have, over the past 50 years, already prevented a total of about 400,000 endometrial cancers before the age of 75 years, including 200,000 in the past decade."

Associate Professor Canfell said women should not decide to take the pill based on the research without speaking to their doctors, but should feel reassured of the pill's safety given the findings.