The upper lip muscles can sometimes elevate so much that your top lip almost disappears when you smile. This gives the appearance of a very prominent gum around the top teeth when these muscles are contacting. It can prevent people from smiling in social situations, in photos and even covering the mouth when speaking and laughing in the company of other people.
A small injection on each side of the lower nose is given to relax this muscle and prevent it from raising the lip so much, allowing it to slightly drop.