The one peptide with respectable human data behind it — all of which was generated by putting it on skin, not into it.
what it is
GHK is a naturally occurring tripeptide — glycine, histidine, lysine — that circulates in human plasma and declines substantially with age. Complexed with copper it becomes GHK-Cu, which has been studied since the 1970s for effects on wound healing and skin remodelling. Copper is not incidental; the complex behaves differently from the bare peptide.
what the evidence shows
Controlled topical studies, mostly small, report improvements in skin firmness, density, fine lines, and photodamage markers. It is an established cosmetic ingredient with a long safety record applied to skin. The mechanistic literature on collagen and extracellular matrix signalling is genuinely substantial.
The one peptide with respectable human data behind it — all of which was generated by putting it on skin, not into it.
what stays uncertain
Injected use has essentially no controlled human evidence. The topical studies say nothing about what happens when the same complex is delivered systemically, and copper homeostasis is tightly regulated for reasons that make casual systemic supplementation a poor idea. Concentration and formulation vary enormously between cosmetic products.
the practical note
This compound illustrates the route problem better than any other. A vendor citing the topical literature to sell an injectable vial is quoting real research about a different thing entirely.