Reviewed November 8, 2026

Lactic Acid vs Glycolic Acid: The 14 Dalton Gap That Decides Your Skin

Glycolic acid weighs 76 grams per mole and lactic acid weighs 90, and that small size gap decides how deep each one goes. The one real head-to-head trial, Stiller 1996, found 8 percent lactic matched 8 percent glycolic on photodamage over 22 weeks, with lactic better tolerated.

Glycolic acid is the acid everyone starts with, and it is the acid a lot of people quit. It works, and it stings, and for dry or sensitive skin the sting is the whole story. Lactic acid is the quieter sibling, and the reason it is gentler is not a different kind of chemistry. It is size.

Both are alpha hydroxy acids. Both loosen the bonds between dead corneocytes so the stratum corneum sheds faster. The physics is where they split: glycolic is the smallest AHA at 76 grams per mole, lactic is larger at 90, and a larger molecule moves through the stratum corneum more slowly. Slower in means less sting, and it also means a different set of side benefits.

The size story, with the numbers

Glycolic acid's pKa is about 3.83 and lactic acid's is about 3.86, nearly identical, so at the same pH the free-acid fraction is close. The difference is the molecule. At pH 3.8, about half the glycolic acid in a formula is present as free acid, and that free acid is what penetrates. Lactic acid at the same pH has a similar free-acid share, but the molecule itself is bigger, so it gets less of itself into the skin per application.

That is why clinical peels at 30 percent and above are usually glycolic rather than lactic. The small molecule reaches deeper, faster, which is what a peel needs. It is also why glycolic burns more at the same strength, and why a 10 percent glycolic toner can feel like too much while a 10 percent lactic feels like nothing is happening.

What the head-to-head trial actually found

Stiller and colleagues ran the comparison that matters: 8 percent L-lactic cream against 8 percent glycolic cream against vehicle, for 22 weeks, on the face and forearms of volunteers at the University of Michigan. On the face, 76 percent of glycolic users and 71 percent of lactic users improved at least one grade on a nine-point photodamage scale. Lactic beat vehicle on more endpoints on the forearm.

Both were well tolerated at those concentrations, which is the part the marketing skips. The trial says the two acids land close on photodamage when they are at the same concentration and the same pH. What changes is tolerance, hydration, and what the molecule does on its way through the skin.

The lactic bonus glycolic does not have

Lactic acid is a natural component of the skin's own natural moisturizing factor, and its hydroxyl group holds water while it exfoliates. Rawlings and colleagues showed in 1996 that L-lactic acid, but not its mirror image D-lactic acid, is incorporated as a substrate into keratinocyte ceramide synthesis, raising ceramide levels in the stratum corneum and improving barrier function. Glycolic acid does not do that.

For dry skin that is the deciding factor. Glycolic acid exfoliates and leaves the barrier to fend for itself. Lactic acid exfoliates while feeding the barrier. The trade is speed: lactic works more superficially, so visible resurfacing takes longer. If your skin is dry and reactive, that trade is worth making.

Picking one for dry skin

Pick lactic if your main complaint is dry, sensitive, or reactive skin, or if you have quit glycolic before because of stinging. Start at 5 to 10 percent, two or three nights a week, and build frequency before you build strength. A well-formulated lactic product sits at 5 to 10 percent for home use, buffered to a pH that keeps the sting down.

Pick glycolic if your main complaint is bumpy texture, dullness, blackheads on an oily T-zone, or keratosis pilaris on the body, where the deeper penetration is doing work lactic cannot reach. Dry skin that picks glycolic anyway should buffer it: apply moisturizer first, wait ten minutes, then the acid, and keep it to twice a week.

Frequently asked questions

Is lactic acid weaker than glycolic acid?

Not weaker, slower. At the same concentration and pH, the Stiller 1996 trial found 8 percent lactic matched 8 percent glycolic on photodamage over 22 weeks. Lactic penetrates less deeply because the molecule is larger, 90 g/mol against 76, so it works more gradually and stings less.

Why does lactic acid suit dry skin better?

Lactic acid is part of the skin's own natural moisturizing factor, so it holds water while it exfoliates, and it feeds ceramide synthesis in the stratum corneum. Glycolic acid exfoliates without those barrier benefits, which is why dry skin often finds it too harsh.

Can I use lactic acid every night?

Build up to it. Start two or three nights a week at 5 to 10 percent, and add frequency only when the skin stays quiet. If moisturizer starts to sting on application, cut back frequency before you change products.

Does lactic acid still need sunscreen?

Yes. All AHAs raise photosensitivity, and lactic acid is no exception, even though it works more superficially than glycolic. Sunscreen stays part of the routine for as long as you use any exfoliating acid, and for a week after you stop.