TL;DR
Polyhydroxy acids (PHAs) are large-molecule chemical exfoliants, mainly gluconolactone and lactobionic acid. They exfoliate the top of the stratum corneum without diving deep, so they cause minimal stinging or photosensitivity. PHAs suit rosacea, sensitised skin, retinoid users on rest nights, and anyone whose AHA tolerance ran out.
PHAs occupy the odd middle ground in chemical exfoliation. They are slower than glycolic. They are gentler than salicylic. They are the acid for people who got told by a dermatologist they cannot tolerate acids. That last group is bigger than the industry pretends, especially as more people develop rosacea or barrier sensitivity in their thirties and forties.
What makes PHAs different
Polyhydroxy acids carry multiple hydroxyl groups on a sugar-derived backbone. The two you will see most: gluconolactone (derived from glucose, oxidised) and lactobionic acid (lactose-based). Their molecular size sits between 180 and 358 Daltons, roughly twice the size of glycolic acid.
That size is the entire point. A larger molecule does not penetrate as deeply or quickly. It loosens dead cells at the surface, hydrates as a humectant (PHAs hold water), and exits the skin before it has time to provoke nerve endings. The result is exfoliation that feels almost like nothing happened, which is exactly what some skin needs. The acid family tree walks through how this compares to AHA and BHA in plain terms.
The actual numbers
A 2004 study in Cutis, summarised by the AAD, tested 8 percent gluconolactone on rosacea-prone skin over 12 weeks. Tolerability was 96 percent (compared to 71 percent for glycolic acid at the same concentration). Roughness scores improved by 35 percent. Erythema (redness) did not increase, which is the rare and useful part for reactive skin.
Another commonly cited figure: PHAs reduce UV-induced damage in vitro at a rate roughly comparable to vitamin C at low concentration, because the polyhydroxy structure scavenges free radicals as it exfoliates. That is a small bonus, not a primary buying reason, but worth knowing.
Where they fit in a real routine
I use a PHA toner four nights a week on the nights I am not using retinol or AHA. It keeps texture moving without overlap. For a friend with rosacea who reacts to almost everything, PHA is the only chemical exfoliant her face tolerates, and even that took an eight-week ramp from twice weekly to four times weekly.
For oily, thick skin used to glycolic, a PHA will feel underwhelming. That is not a failure of the ingredient. It is a mismatch.
The contrarian section: gentle does not mean optional
I see PHAs dismissed in skincare forums as “baby acid for people who cannot handle real exfoliation.” That framing is wrong, and I think it comes from people whose skin has not yet been pushed past its limits.
A daily PHA at 4 to 8 percent does more useful work over six months for sensitive skin than an aggressive weekly glycolic that causes a flare every other week. Consistency beats intensity in chemical exfoliation. Five words: gentler often wins long-term.
Reactive skin should not be running acid trials. Sensitive-skin routines almost always benefit from a PHA before they touch glycolic or lactic.
How to use them
Start with a PHA toner or essence three nights a week. Apply after cleansing, before serums and moisturizer. Build to nightly over four to six weeks if your skin is tolerating it. You can layer PHAs with niacinamide, peptides, and most hydrators. Layering with vitamin C is fine because the pH bands overlap reasonably.
What to avoid: stacking PHA with AHA or BHA on the same night. Even though PHA is gentle, the combined acid load can over-exfoliate sensitised skin. Save the stronger acid for separate nights.
Lactobionic versus gluconolactone
Lactobionic is the larger of the two and has more humectant pull. It is the better choice for very dehydrated, very reactive skin. Gluconolactone is slightly smaller, exfoliates a bit more efficiently, and is the better choice for someone stepping down from glycolic. Both are pregnancy-safe.
If you are mid-retinol-introduction and your barrier is complaining, swapping in a PHA on rest nights can save the routine. How to introduce retinol covers the pairing logic. Adjacent reads live under chemical exfoliation.
The label test
Look for gluconolactone or lactobionic acid in the top half of the INCI list. A common formulation mistake is including 0.5 percent PHA as a marketing claim and calling the product a PHA exfoliant. The clinical effect happens at 3 percent and up.
One more red flag: products that list a PHA alongside a 10 percent AHA. That is an AHA product wearing PHA branding. The acid load is whatever the AHA delivers, and the PHA is a footnote.
FAQ
Are PHAs safe in pregnancy? Yes. No pregnancy-specific concerns and no systemic absorption issues at topical doses.
Do PHAs cause sun sensitivity? Less than AHAs do, but daily SPF is still required.
Can I use PHA with retinol? On alternate nights or layered carefully (PHA, then wait, then retinol). Daily simultaneous use can over-exfoliate.
How quickly will I see results? Smoothness in two to three weeks. Tone evenness in eight to twelve weeks.
Do PHAs help with acne? Mild help with surface congestion. For active acne, salicylic acid is more effective.
Sources: PubMed / Cutis (2004) on gluconolactone in rosacea-prone skin; AAD on alpha- and polyhydroxy acids.