A gloomy mouth with drooping corners can give a grumpy look. Sometimes this is a natural facial trait, but it's usually due to aging. As facial volume decreases, the quality of the skin also diminishes. This is often seen around the mouth. Lifting the corners of the mouth can be done to a certain extent with botulinum toxin.
The muscle that pulls the corners of the mouth down, causing them to droop, is then injected with botulinum toxin. Because the muscle can no longer "pull" on the corners of the mouth, the corner of the mouth straightens up again. This effect lasts up to 3-4 months and must be repeated to maintain the result.