AHA vs BHA vs PHA: Three Names, Two Levels of Certainty

The three families are presented as a ladder of gentleness, with PHA at the mild end. The structural difference between the first two is small and precise. The difference in how well each is documented is much larger, and it does not run in the direction the marketing does.

Two of these have their own pages at the FDA, with published research behind them. The third has no entry in any source checked for this article.

AHA vs BHA vs PHA: what actually differs

The distinction between the first two is a position on a molecule. StatPearls, describing agents used in chemical peels, puts it exactly: In AHAs, the hydroxyl group is attached to the alpha position of the molecule, while salicylic acid, as the representative BHA, features a hydroxyl group connected to the second carbon.

That small difference has a practical consequence, and it is the one worth carrying. Glycolic acid vs salicylic acid comes down to what each dissolves into: Salicylic acid boasts higher lipophilicity than glycolic acid, allowing it to penetrate the skin more effectively, and StatPearls draws the conclusion directly, since This lipophilic property makes salicylic acid an excellent option for acne treatment, as it can effectively permeate acne lesions and oily skin.

Oil-soluble goes where oil is. That is the entire case for using a BHA on a clogged, oily face rather than an AHA, and it is a property rather than a marketing claim.

The AHA family is broader than glycolic acid alone: AHAs include glycolic acid from sugar cane, lactic acid from milk, and citric acid from citrus fruits.

What is a BHA in skincare, on a label? The FDA lists the names it hides behind, and one of them surprises people: BHA ingredients may be listed as salicylic acid or related substances such as salicylate, sodium salicylate, and willow extract, along with beta hydroxybutanoic acid, tropic acid and trethocanic acid. In practice one dominates: Currently, the BHA most commonly used in cosmetics is salicylic acid.

Willow bark extract marketed as the gentle natural alternative to salicylic acid is, by the FDA's own listing, a way of delivering the same family.

One classification is genuinely unsettled, which is worth knowing before you use it to sort a shelf. Of citric acid, the FDA notes that On rare occasions, citric acid is also cited as a BHA in cosmetic formulations. More commonly, citric acid is referred to as an AHA.

What both families do is the same thing. In the FDA's description, AHA-containing products cause exfoliation, or shedding of the surface skin, and how much depends on more than the percentage: The extent of exfoliation depends on the type and concentration of the AHA, its pH, and other ingredients in the product. Percentage alone has never been the whole story, which is the same limitation that runs through how to read a skincare ingredient list. The pH half of that sentence is its own subject, covered in skin pH along with what washing does to the skin's own reading, and the concentration half in glycolic acid percentage, where the published safety conclusion turns out to bind the two together.

AHA vs BHA vs PHA on the sun question

Do AHAs make you sun sensitive? Yes, measurably, and the FDA ran the studies itself rather than citing someone else's.

Its summary of the finding: These studies confirmed previous industry studies indicating that applying AHAs to the skin results in increased UV sensitivity. Two numbers follow. After four weeks of AHA application, volunteers' sensitivity to skin reddening produced by UV increased by 18 percent, and separately, the volunteers' sensitivity to UV-induced cellular damage doubled, on average, with considerable differences among individuals.

Doubled, on average, with considerable individual variation. That is a larger effect than the 18 percent figure that usually travels alone.

Then the part that almost never travels with it, and it changes the shape of the risk entirely. The FDA reports that this increase in sensitivity is reversible and does not last long after discontinuing use of the AHA cream, with a specific interval attached: One week after the treatments were halted, researchers found no significant differences in UV sensitivity among the various skin sites.

So this is a state you are in while using it, not damage you accumulate. Sun protection is the answer the regulator gives, and what the label is and is not required to tell you about that product is set out in physical vs chemical sunscreen. Stop for a week and the measurements come back. The practical instruction is short: If you are using AHAs, it is advisable to use sun protection.

The BHA side is more interesting than a simple contrast, and both halves have to be said together.

A study the FDA ran with the National Toxicology Program found that glycolic acid did not affect photocarcinogenesis (the development of cancer cells associated with exposure to light) in mice, while salicylic acid had a photoprotective effect (protected against the effects of light) in mice.

Photoprotective, in mice, for a cancer endpoint. That is a real finding and it is not a license to skip sunscreen, because a separate body has conditioned its safety verdict on exactly this. On the FDA's BHA page, the Cosmetic Ingredient Review panel's tentative conclusion was that salicylic acid related substances are safe as used when formulated to avoid irritation and when formulated to avoid increased sun sensitivity, and it added that when sun sensitivity would be expected, directions for use [should] include the daily use of sun protection.

Two true things about the same molecule, answering different questions. A mouse photocarcinogenesis endpoint is not the same as whether a given formula raises your sun sensitivity, and the panel wrote its conclusion as a condition on the formulator rather than a clearance for the ingredient.

For where each of these sits against the other actives on this site, the contrast worth holding is that azelaic acid is named specifically as not causing photosensitivity, while retinoids raise it. Acids are the middle case: measurably raised, reversible, manageable with sunscreen.

The third leg

Is PHA gentler than AHA? That is the claim the category is sold on, and I cannot support or refute it from anything I can verify.

Being precise about the limits of that statement. On 30 August 2026 I checked DermNet, which has no polyhydroxy acid topic page; the FDA's cosmetic ingredient section, which has pages for alpha and beta hydroxy acids and none for polyhydroxy acids; and the StatPearls chapter on chemical peels, which names glycolic, lactic, citric and salicylic acids and does not mention polyhydroxy acids, gluconolactone or lactobionic acid at all.

Published research on PHAs does exist. It sits in journals whose pages will not load for an automated check, which under this site's rules means I will not quote it and will not build a claim on it. There is also a Cosmetic Ingredient Review document covering this ingredient group, and it is a draft stamped as distributed for comment only and not to be cited or quoted, so it is not used here either.

What follows from that is narrower than it sounds, so it is worth stating both ways.

It does not mean PHAs are ineffective or unsafe. Absence of an entry in three reference sources is absence of an entry, not evidence of a problem.

It does mean the comparison is not three-way. For AHAs there is a regulator's own study with numbers and a reversal interval. For BHAs there is a named safety conclusion with conditions attached. For PHAs, on the sources checked here, there is nothing at that level to set beside them. If you are choosing on the strength of the evidence rather than on how a product feels, that asymmetry is the finding.

FAQ

What is the difference between AHA and BHA? Where the hydroxyl group sits on the molecule. The practical consequence is solubility: salicylic acid is more lipophilic than glycolic acid and penetrates more effectively, which is why the BHA suits oily and clogged skin.

Do AHAs make you sun sensitive? Yes. FDA studies found UV reddening sensitivity increased by 18 percent after four weeks of use, and sensitivity to UV-induced cellular damage doubled on average. The FDA advises using sun protection with AHAs.

Is that permanent? No. The FDA describes the increase as reversible, and reports no significant difference in UV sensitivity one week after use stopped.

Glycolic acid vs salicylic acid: which should I use? They suit different problems rather than different budgets. Glycolic acid is water-soluble and works on the surface; salicylic acid is oil-soluble and reaches into the follicle, which is why it is the one documented for acne.

What is a BHA in skincare? Most often salicylic acid. The FDA also lists salicylate, sodium salicylate and willow extract among the names a BHA may appear under, so willow extract is not an alternative to it.

Is PHA gentler than AHA? I could not verify that claim. DermNet, the FDA's ingredient pages and the StatPearls chemical peel chapter have no entry for polyhydroxy acids, so there is nothing at the level of the AHA and BHA evidence to compare it against.

Can you use AHA and BHA together? The question of which combinations are real problems and which are repeated rules of thumb is worked through in skincare ingredient conflicts. The more useful constraint is total load rather than any specific pairing.

Does a higher percentage exfoliate more? Not on its own. The FDA names type, concentration, pH and the other ingredients in the product as jointly determining the extent of exfoliation.

Choosing, given what is actually known

If your complaint is congestion in oily skin, the BHA has both a mechanism and a documented use, and salicylic acid covers what the label is required to tell you about strength. If your complaint is surface texture or tone, an AHA is the family with the regulator's own research behind it, and the price of that is a measurable, reversible rise in sun sensitivity that sunscreen answers.

If you are drawn to PHAs because a product felt kinder than the last one, that is a real reason to keep using it. It is not the same as the evidence that exists for the other two, and nobody selling you the third one is going to be the one to say so.