Long-term use of hormonal contraceptives can double the risk of brain cancer, a study has found.
But scientists stressed that the likelihood of any individual woman developing the disease was in reality extremely small.
Researchers used health data from Denmark to compare 317 women diagnosed with glioma, a type of brain tumour, and 2,126 who were free of the disease.
All were aged 15 to 49, and therefore young enough to be using contraceptives.
Women who had ever used an oral contraceptive or hormone-releasing intra-uterine device (IUD), or coil, were 50% more likely to develop brain cancer than those who had not.
The difference in risk increased to 90%, just under double, for women who had used one of the contraceptives for five years or more.
Progestagen-only contraceptives were most strongly associated with glioma, raising the risk almost three-fold.
In terms of cancer type, the risk was greatest for gliobastoma multiforme, the most aggressive type of primary brain tumour.
Danish lead researcher Dr David Gaist , from Odense University Hospital and University of Southern Denmark, said: “It is important to keep this apparent increase in risk in context.
“In a population of women in the reproductive age, including those who use hormonal contraceptives, you would anticipate seeing five in 100,000 people develop a glioma annually, according to the nationwide Danish Cancer Registry.
“While we found a statistically significant association between hormonal contraceptive use and glioma risk, a risk-benefit evaluation would still favour the use of hormonal contraceptives in eligible users.”
The findings are reported in the British Journal of Clinical Pharmacology.
The hormone progesterone is known to increase proliferation of high-grade glioma cells called astrocytomas in the laboratory.
It is also thought to raise levels of growth factors, natural substances that stimulate cell growth.
But the scientists said it was impossible to form any conclusions about how progesterone-containing contraceptives might influence brain cancer development.
Experts commenting on the results emphasised the importance of keeping them in perspective.
Sir David Spiegelhalter, Winton Professor of the Public Understanding of Risk at Cambridge University, said: “Glioma is fortunately very rare.
“Even if oral contraceptives did increase the risk to the extent suggested by this study, it would only mean one extra glioma each year for every 50,000 women taking the pill.
“Suppose, however, all these women changed to a less effective form of contraception and 10,000 of them got pregnant: we would then expect one extra mother and 40 extra babies to die.”
Statistician Professor Kevin McConway, from The Open University, said: “Previous scares about possible increased health risks from contraceptive pills have had bad consequences for public health.
“On hearing that taking the Pill might lead to an increased risk of certain diseases, some women switched away from the Pill to less reliable forms of contraception, or none at all.
“That increased numbers of abortions, and exposed the women to health risks from pregnancy and abortion that were generally far greater than any reduction in risk from the diseases in the scare.
“Brain tumours are not at all common.
“So even if long-term pill use does double the risk, well, twice a very small risk is still a very small risk.”