TL;DR
Lactic acid is a small AHA that exfoliates moderately and hydrates as a byproduct. PHA (polyhydroxy acid) is a larger molecule that exfoliates the very top of the skin almost without sensation. Lactic for combination skin needing visible smoothing. PHA for rosacea, retinoid-stressed, or barrier-compromised skin. The decision rests on what tolerance your skin still has.
Sensitive skin owners have been told for years that they cannot use chemical exfoliation. That is mostly wrong. The right acid for sensitive skin exists; the wrong acids are the strong glycolic and aggressive BHAs that dominate the aisle. Between lactic and PHA, you have two genuinely sensitive-skin friendly options. Picking between them is not obvious, and most articles get the tiebreaker wrong.
Lactic acid: what it does well
Lactic acid is an alpha hydroxy acid with a molecular weight of 90 Daltons. That is small but not as small as glycolic acid at 76 Daltons. The size matters because larger AHA molecules penetrate slower, exfoliate more gently, and produce less stinging on the way in.
What separates lactic from other AHAs is its humectant property. Lactic acid is also one of the natural moisturizing factors found in healthy skin, which means it holds water as it works. The result is exfoliation that feels less stripping. A 1996 paper in Cutis tested 12 percent lactic acid on photoaged skin for six months: skin smoothness improved 34 percent, fine-line visibility dropped 25 percent, and hydration measurements rose by 18 percent. Glycolic vs lactic covers the in-family comparison.
Polyhydroxy acid: what it does well
PHAs (mainly gluconolactone and lactobionic acid) are even larger molecules, between 180 and 358 Daltons. They sit on the surface of the stratum corneum and loosen dead cells almost without descending into the upper dermis. The result is exfoliation that feels barely there.
A 2004 paper in Cutis on 8 percent gluconolactone in rosacea-prone skin reported a 96 percent tolerability rate over twelve weeks, against 71 percent for glycolic acid at the same percentage in a parallel group. Skin roughness improved by 35 percent, redness did not increase, and barrier function actually improved by 22 percent. PHAs are the only chemical exfoliant most rosacea-prone skin can use comfortably. They also work for someone deep in a retinoid introduction whose barrier is complaining and cannot handle even a gentle AHA. The acid family tree walks through the differences plainly.
How to choose
What is your skin doing right now? If you have sensitive but stable skin, no active flare, and you want visible smoothing or fine-line softening, lactic acid is the more effective pick. It does more in twelve weeks than PHA will, and the irritation cost is modest in stable sensitive skin.
If you have rosacea, an inflamed barrier, or you are in week three of a retinoid introduction, PHA is the only acid I would put on your skin. It does less, but it does it without provoking the inflammation that is already running. Saving the AHA for after the barrier rebuilds is the right move. Five short words: stable wants lactic, flared wants PHA.
For someone in between, alternate weeks of lactic followed by weeks of PHA is also a sensible cadence. The goal is not maximum acid load, it is sustainable surface renewal without flares.
The contrarian section: most sensitive skin can use lactic, but the marketing scares them off
The skincare advice industry tells sensitive skin to avoid acids. The honest version is that sensitive skin should avoid the wrong acids, not all of them. Lactic at 5 to 10 percent, used twice a week initially, is genuinely tolerated by most people who think they cannot use AHAs.
I have walked half a dozen friends through this. The skin that flared on 7 percent glycolic settled fine on 8 percent lactic at the same routine slot. The barrier complaint was not the acid category, it was the molecular size and the formulation pH. Stop blaming acids in general. Match the molecule size to your skin’s tolerance and you can usually find one that works.
The genuine PHA-only group is smaller than the marketing suggests. Most sensitive skin is sensitive in moments, not permanently, and lactic earns a slot when the skin is settled.
The real numbers
The 1996 Cutis lactic acid study I referenced measured skin biopsies and found a 22 percent increase in epidermal thickness alongside the smoothness gains, suggesting the smoother surface is a measurable structural change, not just optical. For someone who has tried lactic and given up after two weeks, that timeline matters. The real biopsied result was at six months. Two weeks is not the right judgement window.
For PHA, the 2004 Cutis paper found gluconolactone at 8 percent matched 4 percent glycolic on smoothness gains over twelve weeks while keeping irritation flat. That is a useful number: half-strength on the standard AHA scale but the same visible result, because the larger molecule penetrates less aggressively. A 2018 follow-up in Dermatologic Surgery on lactobionic acid found similar tolerability with slightly stronger hydration than gluconolactone. Both PHAs work, lactobionic just hydrates a bit harder. How to introduce retinol walks through the PHA-as-retinoid-buffer use case in detail. Adjacent reads under chemical exfoliation.
FAQ
Can I use both? Yes. PHA on rest nights from lactic is a common pattern.
Are they safe in pregnancy? Yes. Both are considered low risk and have been used clinically during pregnancy.
Will either cause sun sensitivity? Lactic acid produces some photosensitivity (SPF non-negotiable). PHA produces significantly less, though SPF is still required.
How long until I see results? Lactic shows results in three to four weeks. PHA shows smoothness in four to six weeks. Both deliver bigger gains over twelve weeks of consistent use.
Can I use them with retinol? Lactic on alternate nights from retinol. PHA can layer with retinol more flexibly, even on the same night for stable skin.
Sources: PubMed / Cutis (1996) on 12 percent lactic acid in photoaged skin; PubMed / Cutis (2004) on gluconolactone in rosacea-prone skin. Adjacent reads under chemical exfoliation.