GHK-Cu is one of the few skincare peptides with human data. How good is it?

GHK-Cu is marked evidence: mixed, which puts it above most of the skin category. The page says it is one of the few with human studies rather than only lab work.

How good are those studies, and is 1-2% actually the meaningful range or a number that got repeated?

The evidence is better than most of the category and that is a low bar, so let me be precise about what "mixed" is doing.

There are human studies, they are mostly small, mostly manufacturer-adjacent or manufacturer-funded, and mostly measuring firmness and texture endpoints that are hard to blind well. That is not nothing — the wound repair literature going back to the 1970s is genuinely substantial, and the molecule occurs naturally in plasma and declines with age, which is a real observation rather than a marketing line.

What "mixed" means here: several studies show something, the effect sizes are modest, and the quality is not what you would want.

Which is, honestly, a fair description of essentially all cosmetic actives that are not retinoids or sunscreen.

The formulation problem is the one that makes percentages nearly meaningless on this one.

Copper peptides are notoriously awkward to formulate. The copper is the active part and it is also reactive — it interacts with other ingredients, it is pH sensitive, and it is the reason the serum is blue and the reason it can go off. Two products at the same stated percentage can differ enormously in how much intact GHK-Cu is present when you put it on your face.

So the number on the bottle is an input, not an output. There is no way for a consumer to measure the output.

And the widely repeated advice not to layer it with vitamin C is downstream of that reactivity?

Downstream of the chemistry, yes, although the strength of the advice outruns the evidence for it. The plausible mechanism is real; the "your serum is now useless" framing is stronger than anyone has demonstrated. Separate them if it is easy, do not agonise.

The genuinely useful thing on that page for me was the note that it is gentle enough for skin that cannot tolerate retinoids. That is a real clinical niche rather than a claim, and it is the reason I would consider it — not the firmness numbers.

Agreed, and that is a nice example of the useful information being the boring sentence rather than the headline.