Lip injections are a cosmetic procedure used to increase fullness in the lips by using injectable fillers like hyaluronic acid, fat, or implants. Hyaluronic acid—a naturally occurring substance in the body—is the most common type of filler used to get a plumper pout. The type of filler chosen depends entirely on what your goal is for the treatment, whether it be to balance out the shape of your lips or add fullness or structure.