Some people have downturned mouth corners which makes them look like they are sad. This is a result of depressor anguli oris muscle which pulls the mouth corners downwards. Some can also have associated Marionette lines that extend from the corners of the mouth furthermore downwards.
Small doses of Botox injected into depressor anguli oris muscle can weaken it and allow lifting muscles to raise the mouth corners.
Although Botox injections itself can lift up the mouth corners, it may be beneficial to treat deep lines extending from the corners of the mouth (Marionette lines) with dermal fillers as well.