Great question. Collagen has been replaced by a new generation of fillers for the lips. Collagen was offered for injections in the 1990s but had limited longevity and some allergic risk. They have been replaced by things like hyaluronic acid gels which are easier to inject, longer lasting, and without allergic risk. I would recommend Juvederm Ultraplus which is a smooth gel that I utilize to plump and shape the lips. These fillers are temporary and reversible.
I hope this helps.