It might seem that the question of which injectable to use for the lips would be a simple one. The reason that it isn't is because the lip unit encompasses far more than the red area we loosely call the lips.
The entire lip unit includes the philtrum (the two vertical columns extending upward from the center of the upper lip to the base of the nose, the white roll (the slightly projecting whitish border that rims the red border of the lips, the Cupid's bow, and the degree of projection of the red areas. It also includes considerations, such as the width of the entire white portion of the upper lip, and whether there are bar code lines above and below from aging and sun damage or transverse lines under the nose related to muscular activity in the area.
Today, the situation is fortunately not that of "All we have is a hammer, so everything has to be a nail." In other words, each of the areas above might need a different agent from our injectable "tool box." For example, the red lips might be best treated with a smooth volumizing agent, like Juvederm UltraPlus XC, while the the philtral columns might respond best with a agent, like a Restylane L, a filler with more turgor (more lifting ability). Perlane L, a volumizer with a good lift, might be better suited for the depressed areas under the outer third of the lower lip, to give support to that region and diminish any sagging of the angles of the mouth. And Juvederm Ultra XC might be needed for a finishing touch to the angles of the mouth to give them the final straighening needed for a fresher, younger look.
Finally, if a transverse line is prominent directly below the nasal base, a tiny drop of Botox, a neuromodulator, and not a filler or volumizer at all, may be needed in addition to the other treatments mentioned.