Taking contraceptives that contain a low dose of oestrogen has been linked to a reduced rate of ovarian cancer, in a study of almost two million women.
Until now, it has been unclear whether modern forms of the combined oral contraceptive pill have the same link to lowered risk of ovarian cancer that is known for older forms of the pill, which use a higher dose of oestrogen.
'Based on our results, contemporary combined hormonal contraceptives are still associated with a reduced risk of ovarian cancer in women of reproductive age, with patterns similar to those seen with older combined oral products,' said the authors, led by Dr Lisa Iversen at the University of Aberdeen.
The research analysed the health records of 1.9 million women between the ages of 15 and 49. It focused on data from Danish registries between 1995 and 2014 that included hormone prescriptions as well as other information such as cancer records.
The study found that women who were current or previous users of the pill were almost half as likely to develop ovarian cancer, at 4.3 cases in 100,000 compared with 7.5 in 100,000 for women who had never taken the pill.
The effect was evident even in women who had stopped using the pill more than 12 months ago, although current and more recent users showed the lowest rates of cancer. 'The reduced risk seems to persist after stopping use although the duration of benefit is uncertain,' said the researchers.
The study also showed an increasing positive effect with longer durations of contraception use. Women who had taken the pill for more than a decade were three times less likely to develop the disease.
However, women who took progesterone-only pills did not show similar results. A limitation was that the sample size of women taking the progesterone-only pill was much smaller than that for the combined pill, making up just 14 percent of the records.
The study excluded women who were had previously had cancer or were infertile and adjusted for factors such as age, education level and family history of disease. It did not consider the effects on women older than 49.
'[The study] shows that the combined pill which is thought of by patients and GPs as the one with more risks actually has some benefits too, over [progestin-only] methods,' Dr Channa Jayasena, a reproductive endocrinologist at Imperial College London, who was not involved in the research, told the Guardian.
The research was published in the BMJ.