Botox injections may be designed to reduce wrinkles but they also may leave you feeling blue.

Cosmetic injections of botox for crows’ feet around the eyes may cause feelings of depression, according to a British researcher. Why? Because these injections impact the strength of the eye muscles, which are essential in the face’s overall formation of a smile.

The small study, led by Dr. Michael Lewis of the School of Psychology, Cardiff, Wales, involved 25 people who had received Botox for wrinkles and examined how their facial expressions produce, as well as reflect, emotions because they reinforce them.

Lewis said it all boils down to this: people smile when they are happy and smiling can make a person happy.

“Treatment with drugs like Botox prevents the patient from being able to make a particular expression,” he said. “The new finding being reported [this week] concerns the impact of treatments for crows’ feet. The muscles around the eyes are used when forming a real smile and so it was predicted that treatment of the muscles that cause these will reduce the strength of a smile.”

With the help of a questionnaire, Lewis found that those people who had a harder time smiling reported greater feelings of depression.

Previous research has found that when people smile — even if it’s a fake smile — they actually feel less stress and happier in general.