The study's author, Dr. Angelica Lindén Hirschberg, professor at Karolinska University Hospital, told BuzzFeed Health via email: "We do not know the mechanism, but we think it could be a direct effect of the progestin component on the brain."
That's not a bad theory, said Dr. Mary Jane Minkin, OB-GYN and professor at Yale School of Medicine. All different types of birth control pills contain the same type of estrogen, but it's the progestin that varies from brand to brand, she said. And some people may react differently to different progestins. It explains why your best friend might love the same pill that made you moody and nauseous. So it's possible that some of the women in this study reacted negatively to the progestin in this pill, which was levonorgestrel.
"When I [have a patient] go on a birth control pill, I really try to tailor the birth control pill to the patient," said Minkin.