The contraceptive pill substantially reduces women's risk of ovarian cancer and continues to protect them for at least 30 years after they stop taking it, according to a big scientific analysis published today.

The research, by Oxford University epidemiologists, suggests that the risk of ovarian cancer is cut by 20% for every five years that a woman has been on the pill. Those who take it for 15 years cut their risk by half. Ovarian cancer is hard to detect and kills two-thirds of those who get it.

The public health implications are great, say the scientists. The pill has prevented about 200,000 cases of ovarian cancer and 100,000 deaths around the world - mostly in the developed countries - over the last 50 years, they say. Because women get ovarian cancer later in life, often around 55 and onwards, those numbers will rise as the preventive effect kicks in for women who took the pill years ago. Writing in the Lancet medical journal, Professor Valerie Beral and colleagues say they expect the number of cases prevented to rise to 30,000 a year.

The large review, which included data from 45 studies involving 100,000 women, will reopen the arguments over the pill. Women deserted it in droves years ago because of fears that it might increase their breast and cervical cancer risks, but recent work has shown the increase in risk occurs as women get older, beyond the age of around 35. It also disappears almost as soon as they stop taking the pill.

The new findings on ovarian cancer suggest that it could be beneficial for young women to take the pill for 10 years or so. An editorial in the Lancet, one of the world's leading medical journals, calls for the pill to be made easily available.

"Women deserve the choice to obtain oral contraceptives over the counter, removing a huge and unnecessary barrier to a potentially powerful cancer-preventing agent," it says.

Why the pill should prevent ovarian cancers is unknown.

Two Canadian scientists, writing a commentary for the Lancet, say the study shows "that this unequivocal protective effect stems from the cumulative suppression of ovulatory cycles". It appears that oral contraceptives in young women "could help to decrease the number of cells that are likely to become malignant over a lifetime, whereas HRT after menopause could have a carcinogenic effect," write Eduardo Franco and Eliane Duarte-Franco from McGill University and the Institut National de Santé Publique du Québec.

The results of the Oxford study are "unequivocal good news", they say, but add that women and their doctors will once more have to perform a balancing act between the risks and benefits of the pill.

The scientists gathered data from 23,257 women who had developed ovarian cancer and 87,303 who had not. Some 31% of the first group had used the pill and 37% of the second.

They found that in high-income countries such as the UK, where 25% of women aged 16-49 take the pill, 10 years of oral contraceptives reduced the numbers getting ovarian cancer from 12 per 1,000 to eight, and their chance of death from seven per 1,000 to five.

The Family Planning Association said it was great news for women. "There is now substantial evidence showing that for most women the benefits of taking the contraceptive pill are far greater than any of the risks," said the association's chief executive, Julia Bentley.

Backstory

The pill is credited with kick-starting the sexual revolution and putting the swing into the 1960s. But in the 50 years of its existence, the pill has provoked as much angst as free love. It was developed in the US and first tested by women in Puerto Rico. Early versions contained high doses of oestrogen and progestin, the hormones involved in women's reproductive cycle. By 1969, one million women were on the pill. Now there are 3.5 million on it in the UK, and 100 million worldwide. Scares in the 80s over links to breast cancer, heart attacks, strokes and blood clots dented the pill's popularity. Low dosage formulations have since reduced many of the risks.