Taking hormonal contraceptives, such as the pill, for at least five years may up women's risk of developing a rare brain tumour, scientists have warned.
Hormonal contraceptives, including oral contraceptives, contain female sex hormones and are widely used by women all over the world.
While only a little is known about the causes of glioma and other brain tumours, there is some evidence that female sex hormones may increase the risk of some cancer types, although there is also evidence that contraceptive use may reduce the risk in certain age groups, researchers said.
"This prompted us to evaluate whether using hormonal contraceptives might influence the risk of gliomas in women of the age range who use them," said research team leader Dr David Gaist of the Odense University Hospital and University of Southern Denmark.
Researchers drew data from Denmark's national administrative and health registries, enabling them to identify all the women in Denmark who were between 15 and 49 years of age and had a first-time diagnosis of glioma between 2000 and 2009.
They found 317 cases and compared each of these women with eight age-matched women who didn't have gliomas.
"It is important to keep this apparent increase in risk in context," said Gaist.
"In a population of women in the reproductive age, including those who use hormonal contraceptives, you would anticipate seeing 5 in 100,000 people develop a glioma annually, according to the nationwide Danish Cancer Registry," Gaist added.
"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," said Gaist.
The study was published in the British Journal of Clinical Pharmacology.
