Both collagen and fat injections are used to fill depressions or lines in the skin. Collagen is a normal component of all skin; however, the material used for an injection is prepared from calfskin. Because this collagen is a foreign protein, patients must be skin tested to see if they are allergic to this material. The collagen is injected into both the deeper and shallow layers of the skin in order to fill up a depression or line. This is done in a doctor's office and the patient can wear make up immediately and return to work without the procedure being noticeable. Depending on the area injected, there can be mild discomfort during the injection process. Because the collagen is gradually broken down by the body, collagen injections provide only temporary improvement; typically, three to six months. Another substance that can be injected or transplanted beneath lines or depressions is the patient's own fatty tissue, which holds no risk of allergy or rejection. However, much of the fat that is injected in the area does not survive the transplant process, so more than one injection is usually required. After a series of injections, the correction can be quite satisfactory and long lasting. The injection of fat usually results in more swelling, discomfort and bruising than with collagen. For more information, contact a cosmetic or plastic surgeon.