Millions of women worldwide rely on birth control pills as a safe and effective way to prevent pregnancy. Now, a new study suggests that the hormones in some of these pills may also influence eating behaviour in subtle but important ways, offering fresh clues about how synthetic hormones interact with the brain.

The research, published in the JAMA Network Open, followed 422 women already taking monophasic combined oral contraceptive pills over 49 consecutive days. Instead of comparing different groups, researchers observed the same women day after day, allowing each participant to act as her own comparison. This approach helped the team examine whether emotional eating changed between hormone-containing "active" pill days and hormone-free "inactive" pill days.

"Because we tracked the same women day to day, we could see how eating changed with hormone exposure," said Dr Shaunna Clark, associate professor of psychiatry and behavioural sciences at Texas A&M University's Naresh K. Vashisht College of Medicine.

The findings were striking. Emotional eating—simply explained as eating more than usual in response to stress or difficult feelings—was consistently higher during active pill days. The pattern appeared across two complete pill cycles, among all 422 participants, and even among women already diagnosed with binge-eating disorder. Researchers also found that the effect remained after accounting for negative mood.

"That tells us the hormones themselves may be playing a role, rather than those changes being driven by mood or other factors," Clark said.

The study, led by Dr Kelly Klump of Michigan State University, adds to earlier research showing that binge-related eating often rises after ovulation, when natural levels of oestrogen and progesterone increase. The new work suggests that synthetic hormones in birth control pills may produce similar effects.

However, the researchers stress that the study does not prove birth control pills cause binge eating. It identifies an association, not a cause-and-effect relationship. Individual responses also varied considerably, and the study found little evidence that pill type meaningfully changed body-image concerns or overall mood.

"These findings show a pattern at the group level, but individual responses can vary," Clark noted. "Findings like these can help us better understand how different hormone exposures affect eating behaviour. Over time, that could help clinicians and patients make more informed decisions about care."

Experts say the findings should encourage informed conversations rather than alarm. Women concerned about changes in appetite or eating habits should discuss them with their healthcare provider rather than stopping contraception on their own. As scientists continue to explore how hormones affect the brain, studies like this may help personalise contraceptive care without undermining the well-established safety and effectiveness of birth control pills.