GHK-Cu is a three amino acid peptide, glycine linked to histidine linked to lysine, carrying a bound copper ion. It occurs naturally in human plasma and its concentration falls with age, which is where the anti-ageing framing originates. In skin research it appears to influence collagen production and to have antioxidant activity, and it has a long and legitimate history as a cosmetic ingredient under the name copper tripeptide-1. That history matters, because it is the source of nearly all the credible evidence and it is entirely about topical use on the skin surface. The injectable version circulating in biohacking communities sits outside that framework completely. Cosmetic regulation is not drug approval, and evidence that something helps skin when applied to skin says nothing about what happens when it is introduced systemically.