There are two types of collagen that can be injected as implants: human and bovine. Since the human collagen has to be donated by oneself or by a fresh corpse, cow collagen seems slightly more appealing (although there is a 3% to 7% liklihood of an allergic reaction).
Anyway, even if you get past the issue of origin, there is the issue of longevity. Collagen implants can last as little as one month and usually up to three months. However, there are instances where it can last 18 months. So what impacts this?
Collagen is metabolically broken down over time by enzymes. The denser the collagen is (Zyderm, the bovine filler, comes in several grades for fine lines through to deep furrows), the longer it may last. However the Journal of Drugs in Dermatology says the longevity of injectibles "depends heavily on the quality of the patient's skin, the type and severity of defect being treated, the patient's age and the injection technique utilized."
Top ups are, therefore, regularly required. However, injections of Zyderm (the bovine filler for fine lines) and Zyplast (for deeper furrows) should be limited to 30ml during a year.
All in all, this is not a particularly efficient or cost-effective treatment.