Some users of Mirena birth control have reported suffering from unusually painful headaches.

These headaches could be a sign of pseudotumor cerebri, also known as intracranial hypertension – a condition reported by many patients in conjunction with their use of Mirena.

Mirena is a type of intrauterine device that delivers a dose of the contraceptive drug levonorgestrel. It’s a T-shaped implant that, once placed inside the uterus, can prevent pregnancy for up to five years.

Women in Europe have been using Mirena since 1991, and it’s been available in the U.S. since being approved by the FDA in 2000. Today, it’s estimated around two million women worldwide use Mirena.

The device has been the subject of tens of thousands of reports to the FDA of various complications. Mirena users have reported adverse events like expulsion or dislocation of the device and hemorrhage of the vagina.

One serious development that has been associated with contraceptives like Mirena is intracranial hypertension, an increase in pressure within the skull that puts pressure on the brain and central nervous system.

Mirena Study Finds Higher Rate of Intracranial Hypertension in Mirena Users

In one recent Mirena study, researchers found more reports than expected of intracranial hypertension in Mirena users.

Researchers set out to measure the risk of intracranial hypertension that could be expected in conjunction with use of Mirena. They reviewed reports of Mirena-related complications lodged with the FDA’s Adverse Event Reporting System.

For comparison to other contraceptives, the Mirena study included a retrospective cohort study that analyzed records of patients who had taken the contraceptive medications EE-norgestimate and EE-norethindrone.

According to the Mirena study, the FDA database contained a higher-than-expected number of reports of intracranial hypertension in patients who used Mirena.

The cohort study revealed that the incidence of intracranial hypertension found in Mirena users was similar to that in users of EE-norgestimate

The study was published in the June 2015 issue of the journal Therapeutic Advances in Drug Safety.

Intracranial hypertension is sometimes called pseudotumor cerebri, since the increase in pressure inside the skull can mimic the effects of a brain tumor.

This condition has been reported in association with other types of birth control that also used levonorgestrel, the active ingredient in Mirena.

Scientists aren’t positive what causes intracranial hypertension, but it’s believed to result from an increase of cerebrospinal fluid. Normally, this fluid helps to cushion the central nervous system.

But an excess of fluid could increase pressure on the brain. The result can lead to symptoms like headaches, dizziness or nausea. Some patients report a whooshing sound in their ears. The increased pressure may also cause abnormalities of vision like blurriness, double vision, or blind spots.

FDA: Bayer’s Promotional Materials Downplay Mirena Complications

Bayer, the manufacturer of Mirena, has already gotten some negative attention from the FDA for downplaying the risks of complications associated with Mirena.

In 2009, the FDA notified Bayer that its advertising for Mirena was allegedly false or misleading. The agency said Bayer’s promotional materials, which claimed that Mirena users would “feel great,” failed to address reports of side effects like acne, back pain and weight gain.