Cosmetic injections to treat crows' feet prevent faces showing happiness, a Cardiff University psychologist suggests.
The treatment, which uses the Botulinum toxin, reduces the strength of eye muscles which are key to smiling.
As well as happiness making you smile, smiling makes you happy, he said.
The small study of 25 people looked at the concept of facial feedback - where the expressions we make on our faces affect the emotions we feel.
Dr Michael Lewis, reader at the school of psychology in Cardiff, is presenting his findings to the British Psychological Society's annual conference in Harrogate.
He says that reducing wrinkles by using injections of Botulinum toxin A can affect the way we feel and even how we see the world.
His research looked initially at the psychological impact of reducing frown lines on people's foreheads using the procedure.
"If you can't frown any more you end up feeling happier," he said.
"It's called facial feedback. The expressions we make on our face are connected to the emotions we feel."
Facial feedback, he said, also had an impact on Botox treatment of crows' feet or laughter lines around the eyes.
Using a one-off questionnaire, Dr Lewis assessed the levels of depression in a small group of people who had received this treatment by referring to clinical diagnosis criteria.
He said the effects were significant. Feelings of depression were higher in the group which had treatment for crows' feet and frown lines compared to another group who just had injections for frown lines alone.
He suggested this was because the treatment "reduced the strength of a smile" and made people feel less happy.
"Treatment with drugs like Botox prevents the patient from being able to make a particular expression."
"For example, those treated for frown lines are not able to frown as strongly. This interrupts the feedback they would normally get from their face and they feel less sad."
Dr Lewis now wants to research the effects of similar drug treatments on the emotion of disgust, which is a feature of people with obsessive compulsive disorder (OCD).
If the facial expression of disgust can be reduced then perhaps feelings of disgust can too, he said.
The Royal College of Surgeons advises that only trained doctors, nurses and dentists should provide non-surgical cosmetic treatments such as Botox.
Update 25 July 2013: This story was published on 11 April 2013 but was inadvertently updated with a later datestamp.