Botox treatment has an uplifting side effect that brings a smile to the face, researchers have found.
By preventing frowning, the treatment may actually make people happier, psychologists said.
Scientists studied 25 people, 12 of whom had botulism toxin A injections to the forehead. The others had fillers, peals or cosmetic treatments.
All the participants completed a mood questionnaire at least two weeks later.
Patients who received botox treatment were found to be significantly less depressed, anxious and irritable than those who did not.
Study leader Dr Michael Lewis, from the University of Cardiff, said: "Both groups had had some form of cosmetic treatment, and there was no difference in how effective they thought their treatment had been, so this result is most likely due to the effects of botox specifically."
Botox injections paralyse the forehead muscles, preventing frowning.
The research findings support the theory that expressions feed back and impact emotions, Dr Lewis said.
By blocking frowning, botox prevents expressions of negative emotion, resulting in a happier mood.
"This research may help the development of a new treatment for depressive illnesses," said Dr Lewis, who was presenting the research at the British Psychological Society's annual meeting in Brighton.
"Unlike other treatments for depression, which have significant negative side effects, the main side effect of a botox-based treatment would be a younger-looking face. But as the cosmetic effect of botox is temporary, so will be the emotional effect. As the effect of the botox wears off, one's mood is likely to return to normal levels."
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