When the face reads annoyed at rest, this may be the reason.
When the groove running from the mouth corners toward the chin deepens, the corners look downturned and the expression reads unhappy. Filling this groove with hyaluronic acid softens the line. The area around the mouth moves a lot, so product choice and amount matter.
One session lasts nine to eighteen months, varying by product. The mouth area moves constantly with speech and smiling, so the filler is easily compressed and dispersed and absorbs faster than the deep mid-face. Rather than counting on a long duration, it is topped up as the result is followed.