Certain types of contraceptives could increase seizure risk in women with epilepsy, a new study suggests.

Epilepsy is a neurological disorder caused by malfunctioning nerve cells in the brain. Over a lifetime, one in 26 people will be diagnosed with the disorder, and men and women are at equal risk.

Researchers decided to look further into a previous study that found women using hormonal contraceptives have reported 4.5 times more seizures than those who did not. They also knew that epilepsy is more difficult to control in women once they reach sexual maturity because the hormones that control menstruation and pregnancy can trigger seizures.

“Women often experience more seizures in the week before their period due to the change in these hormones,” says Dr. D. Samba Reddy at the Texas A&M Health Science Center College of Medicine, the study’s lead investigator

His team decided to use an animal model of epilepsy to look at the effect of ethinyl estradiol, which is a powerful synthetic estrogen derivative that some contraceptives contain. 

The study found that when epileptic animal models received ethinyl estradiol, they not only had more frequent seizures, but the seizures were more likely to be uncontrolled. "We suspected for some time that hormonal birth control increases seizure activity in women with epilepsy, but now we know what part of the contraceptive is problematic, says Reddy of the study, which appears in Epilepsy Research.