Does GHK-Cu Actually Penetrate Skin?

A reasonable objection to every topical peptide: the outer layer of skin exists specifically to keep things out. So does this one get in?

Probably yes, on size. That is a smaller claim than it sounds and it is worth being precise about why.

The 500 Dalton rule

The common heuristic in dermatology is that molecules above roughly 500 Daltons struggle to cross the stratum corneum in meaningful amounts. It is a rule of thumb rather than a physical law, and there are exceptions in both directions, but it is a reasonable first filter.

GHK, the tripeptide, has a molecular weight of about 340 Da. As the copper complex, GHK-Cu is around 400 Da. Both sit comfortably under the threshold.

For comparison, the collagen fragments some products advertise run into the tens of thousands of Daltons. Those genuinely do not get anywhere, which is where a lot of justified peptide scepticism originally came from. GHK-Cu is a different size class entirely.

Why size is necessary and not sufficient

Being small enough gets you past the first filter. It does not mean the molecule arrives intact, in the right place, at a useful concentration.

Three other things decide that.

The vehicle. An aqueous base delivers a water-soluble peptide differently from a silicone or oil-heavy one. A heavy occlusive film can trap an active on the surface rather than help it through. This is the most underrated variable in the category.

Stability at the point of contact. The peptide has to still be a copper complex when it lands. At low pH the copper dissociates and you are applying a peptide with no payload. This is why formulation pH is worth publishing and why we do.

Concentration. A molecule can be perfectly sized and perfectly stable and still be present at a dose too small to signal anything. See what percentage actually matters.

Where the evidence actually is

Being straight about this: most of what is known about GHK-Cu comes from cell culture and ex-vivo tissue work rather than from human penetration studies with instrumented endpoints. The molecule behaves as expected in those models, and the size argument is sound, but neither is the same as a measured concentration in living human dermis after topical application.

We counted the whole clinical literature and published the result, including the parts that do not flatter us: three trials, one in rabbits, zero meta-analyses.

What about microneedling to force the issue

It does bypass the barrier, and that is exactly the problem. Cosmetic formulations are tested against intact skin, and a serum going through open channels is a different route of administration with different rules. Why timing is the whole answer.

The short version

At roughly 340 Da, GHK is well under the size threshold where penetration becomes implausible, which is more than most peptides in skincare can say. Size is the easy hurdle. The vehicle, the pH and the concentration are the ones that actually decide whether anything happens.

Related: GHK-Cu explained and do copper peptides actually work. Everything indexed: the copper peptide guide.