Glycolic acid

Glycolic acid is the smallest alpha-hydroxy acid, which lets it penetrate faster than lactic acidLactic acidModerateLactic Acid · Alpha-hydroxy acidGentler, larger AHA that also hydrates; good for texture in drier or more reactive skin.Open in the Index → and makes it both more effective and more irritating at a given concentration and pH.

The evidence

The core trial is a 22-week, double-blind, vehicle-controlled comparison of 8 % glycolic and 8 % lactic acid creams in photodamaged skin. Both improved overall severity, roughness and mottled pigmentation versus vehicle[1].

pH matters more than percentage

Free acid — the fraction actually doing the work — depends on pH. Regulators have converged on ≤ 10 % at pH ≥ 3.5 for consumer products[2][3]; a 10 % product at pH 3.5 is a very different thing from 10 % at pH 4.5. Alternate with retinolRetinolStrongRetinol · Vitamin A alcoholThe best-evidenced over-the-counter retinoid for photoageing; now capped at 0.3 % in EU leave-on products.Open in the Index → rather than layering.