Filler is a hyaluronic acid injectable gel. Hyaluronic acid naturally occurs in the skin to add volume and hydration. Collagen production diminishes with age along with the hyaluronic acid of the skin. Filler is most commonly used for marionette lines (peri-oral lines), smile lines (also known as nasolabial folds), and lip augmentation. Filler generally lasts 6 to 12 months or more, depending on the patient, type used, and the area being injected.