What are polyhydroxy acids?
PHAs are chemically related to the alpha hydroxy acids but carry multiple hydroxyl groups on a substantially larger molecule. The two you will encounter are gluconolactone and lactobionic acid. NeoStrata pioneered their use in skincare, which is why the brand is so closely associated with sensitive-skin exfoliation.
Molecule size is the whole story. Glycolic acid is the smallest AHA, which is why it penetrates fast and works quickly — and why it stings, reddens and overwhelms compromised skin. A PHA molecule is too large to rush in, so it works at the surface, gradually, with far less provocation.
How they differ from AHAs
The exfoliating mechanism is broadly the same: loosening the bonds between dead surface cells so they shed, revealing smoother, brighter skin. The difference is pace and tolerability. PHAs deliver a similar destination over a longer road, which is precisely what reactive skin needs.
PHAs also do something AHAs do not: they are humectants, drawing and holding water in the skin. So rather than leaving skin tight and stripped after exfoliation, they leave it better hydrated. For a barrier that is already struggling, that combination is unusually well suited.
They carry antioxidant activity as well, and unlike glycolic acid they do not appear to increase UV sensitivity to the same degree — though daily SPF 50+ remains non-negotiable with any exfoliant.
Who they suit
PHAs are the sensible starting point for anyone with rosacea, eczema-prone or genuinely sensitive skin, and for anyone who has tried glycolic acid and found it intolerable. They are also useful for a compromised barrier — over-exfoliated, over-treated skin that still needs gentle renewal while it recovers.
They suit deeper skin tones well for the same reason they suit sensitive skin: less inflammation means less risk of post-inflammatory hyperpigmentation. For Fitzpatrick IV–VI skin nervous about acids, a PHA is a far lower-risk introduction than a glycolic peel.
The trade-off is honest: results come more slowly. If your skin tolerates glycolic acid comfortably and you want faster resurfacing, an AHA will get you there sooner. PHAs are about making exfoliation possible for skin that otherwise could not have it.