More than 100 million women around the world use oral contraceptives. Over the years, research has been able to explore and identify much of their physical effects, but what about their psychological effects?

Dr. Alexander Lischke of the University of Greifswald, Germany, notes how "remarkably little" is known about the effects of birth control pills on emotion, cognition, and behavior. 

As part of a new study, published in Frontiers in Neuroscience, Lischke and his research team recruited 95 healthy women — 42 of whom were on the pill and 53 who were not — for an emotion recognition task. The aim was to find out if using the pill could have any sort of impairment on their ability to recognize emotions.

"We assumed that these impairments would be very subtle, indicating that we had to test women's emotion recognition with a task that was sensitive enough to detect such impairments," he explained. "We, thus, used a very challenging emotion recognition task that required the recognition of complex emotional expressions from the eye region of faces."

Both groups of women had no problem in recognizing basic expressions like those of happiness or fear. But when it came to more complex emotional expressions like pride or contempt, women on the pill were 10 percent less accurate compared to their counterparts.

Despite a clear difference, the researchers emphasized that this effect is still quite subtle. While the test was designed in a way to detect even slight differences, it is not yet clear whether this can actually impact the way oral contraceptive users socialize and maintain relationships.

According to the study, the effect was also consistent with both positive and negative expressions and was not influenced by the type of pill or the menstrual cycle phase of non-users.

"Cyclic variations of estrogen and progesterone levels are known to affect women's emotion recognition, and influence activity and connections in associated brain regions," Lischke said. "Since oral contraceptives work by suppressing estrogen and progesterone levels, it makes sense that oral contraceptives also affect women's emotion recognition." 

The new study also strengthens previous experiments that arrived at similar findings —  for example, this study which was published in the journal European Neuropsychopharmacology back in 2016.

With that being said, more research is needed to determine exactly how birth control pills could impair the ability to recognize complex expressions. It is also important to replicate these findings thoroughly before experts can consider changing guidelines on the prescription of birth control pills.

If you want to go on the pill, it is important to discuss your medical history and lifestyle with your doctor who can help you pick the right kind. For instance, women who smoke are usually advised to take progestin-only pills rather than combination pills.