The risk for developing a rare form of brain cancer known as glioma appears to go up with long-term use of hormonal contraceptives such as the Pill, new Danish research suggests.

Women under 50 with a glioma "were 90 percent more likely to have been using hormonal contraceptives for five years or more, compared with women from the general population with no history of brain tumor," said study leader Dr. David Gaist.

However, the Danish study couldn't prove cause-and-effect, and Gaist stressed that the findings "need to be put in context" for women because "glioma is very rare."

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How rare? Only five out of every 100,000 Danish women between the ages of 15 and 49 develop the condition each year, according to Gaist, a professor of neurology at Odense University Hospital. He said that figure includes women who take contraceptives such as the birth control pill.

So, "an overall risk-benefit evaluation favors continued use of hormonal contraceptives," Gaist said.

The findings were published online in the British Journal of Clinical Pharmacology.

In the study, Gaist's team looked at government data on all Danish women between the ages of 15 and 49 who had developed a glioma between 2000 and 2009.

In all, investigators identified 317 glioma cases, among whom nearly 60 percent had used a contraceptive at some point. They then compared them to more than 2,100 glioma-free women of similar ages, about half of whom had used contraceptives.

Use of the Pill or other hormonal contraceptive did appear to bump up the risk for glioma, the researchers reported, and the risk seemed to rise with the duration of use.

For example, women who had used any type of hormonal birth control for less than one year had a 40 percent greater risk for glioma compared with non-users. And those who had used the drug for five years or more saw their risk nearly double compared to non-users, the findings showed.

In addition, Gaist's team found that glioma risk seemed to go up most sharply for women who had used contraceptives containing the hormone progestogen, rather than estrogen.

Dr. Evan Myers is a professor of obstetrics and gynecology at Duke University Medical Center in Durham, N.C. He described the Danish study as "really well-done."

However, he stressed that the study couldn't prove a cause-and-effect relationship between hormonal contraception use and risk for glioma. Myers also suggested that future research focus on a number of indirect factors -- such as the progesterone found in some types of IUDs (intrauterine devices) -- that might also play a critical role in driving up glioma risk.

And in the end, "even if hormonal contraception does increase the relative risk of glioma, the absolute risk -- the actual increase in the chances of having a glioma diagnosed -- is quite small," Myers stressed.

According to his own statistical breakdown, Myers said that between 2000 and 2011, glioma affected less than two out of every 100,000 American women between the ages of 15 and 29.

"To put that in perspective," he said, "that's about one-tenth the risk of death from trauma in women aged 15 to 44, and a little over twice the risk of dying from a complication of pregnancy."

Myers said his number-crunching suggests an even lower risk profile when looking specifically at women who are taking the Pill or another form of hormonal contraception.

"Without going through the math, it's about 8.5 [cases of glioma] per million" for that subset of women, Myers said.