Glycolic vs lactic acid: which gentle acid suits your skin?

Glycolic acid and lactic acid sit side by side on the shelf, often in bottles that look almost the same. Both are alpha hydroxy acids (AHAs), water-loving acids that loosen the dead cells on the surface of your skin so they shed. But they don’t behave identically, and picking the right one can be the difference between smoother skin and a week of stinging.

Same family, different sizes

AHAs are small organic acids. The US FDA names glycolic and lactic acid as the two most common ones in cosmetics. They work the same basic way: a 2018 review in the journal Molecules explains that AHAs weaken the “cohesion” between dead cells in the outer layer, which is the scientific way of saying they loosen the glue so old cells let go.

The main difference is size. Glycolic acid is the smallest AHA, and lactic acid is a bit larger. A 2024 clinical and legal review of AHAs puts the practical result plainly:

• Glycolic acid’s small size lets it absorb more deeply, so it works harder, but that same depth can irritate sensitive skin.
• Lactic acid penetrates less deeply, which reduces the risk of irritation.

Think of it as the same tool with a different grip. Glycolic reaches further; lactic stays closer to the surface.

Lactic acid’s extra trick: hydration

Lactic acid isn’t just an exfoliant. Your skin already makes a version of it. The 2018 review notes that lactic acid, in the form of sodium lactate, is a well-known part of the skin’s natural moisturizing factor, the mix of small water-holding molecules inside your outer skin cells.

That’s why lactic acid shows up so often in body lotions and in products for rough, dry skin. It exfoliates and acts as a humectant, an ingredient that pulls water into the skin at the same time. The 2024 review describes lactic acid as favoured for dry skin because of these hydrating properties, and the 2018 review cites older work finding it less irritating than glycolic acid.

Glycolic acid doesn’t have the same moisturizing reputation. Its strengths are texture, tone and the look of fine lines, which is why it’s popular in toners and facial peels.

Who each one suits

There’s no winner, only a better match. Based on the 2024 review:

• Glycolic acid tends to suit normal to oily skin that tolerates actives well, and people after smoother texture or a more even tone.
• Lactic acid tends to suit dry skin, sensitive skin, and anyone trying an acid for the first time.

A few practical notes:

• If you’ve ever had stinging from a glycolic toner, lactic acid is a reasonable next try rather than giving up on AHAs altogether.
• If your skin is oily and you mostly want to deal with clogged pores, neither is the ideal tool. Both dissolve in water, not oil, so they work mainly at the surface.
• On eczema flares, sunburn or broken skin, skip both until the skin has healed.

Why percentage and pH both matter

The number on the front of the bottle is only half the story. An acid’s strength on your skin depends on how much of it is there and on how acidic the whole product is, which is measured as pH (lower pH means more acidic).

The 2018 review gives a sense of how much concentration changes things: roughly, 1 percent AHA can shift the acidity of the outer three layers of dead cells, while 10 percent can affect all 10 to 20 layers. The review’s blunt conclusion is that whether an AHA is a friend or foe of your skin depends on its concentration.

The 2024 review also shows the high end. In studies of in-clinic glycolic peels, a 50 percent peel caused much more moderate to severe redness, peeling and a pulling sensation than 20 to 30 percent peels, and it warns that aggressive peel combinations can leave dark marks on medium and darker skin tones.

That’s the reason regulators set limits on both numbers, not just the percentage.

What Canada allows

In Canada, Health Canada’s Cosmetic Ingredient Hotlist sets the rules for AHAs in cosmetics. For products you use on your skin at home:

• 3 to 18 percent total AHA, at a pH of 3.5 or higher.
• Between 18 and 30 percent, or a pH between 3.0 and 3.5, is for professional use.
• Labels must say to use only as directed, avoid the eyes, stop and see a doctor if irritation persists, and cover treated skin with sunscreen before going in the sun.

For comparison, the US FDA points to an expert panel that considers AHA products safe at 10 percent or less with a pH of 3.5 or higher. So a Canadian shelf product can legally be stronger than that. A gentle starting point is well below the Canadian maximum.

The sun question

Both acids make your skin more sensitive to ultraviolet (UV) light. The 2018 review cites a study where 10 percent glycolic acid used for 12 weeks increased the skin’s sensitivity to UV and raised the number of sunburn cells, damaged cells that form after UV exposure. Lab work in the same review suggests lactic acid can also amplify some UV-triggered reactions in skin cells.

The FDA notes the effect reverses within about a week after stopping. In practice:

• Wear sunscreen every morning while you use either acid, Canadian winter included.
• Keep going for a week after you stop.
• Don’t start a new acid right before a beach holiday or a ski trip.

How to start

• Choose one acid. Lactic if your skin is dry or reactive, glycolic if it’s resilient and you want more texture change.
• Use it two or three evenings a week at first, on clean, dry skin, then moisturize.
• A mild tingle that fades in a minute is common. Burning, lasting redness or peeling means stop for a few days and restart less often.
• Don’t layer it with a scrub, a retinoid or another acid on the same night.

You can browse the panel’s reviewed exfoliators if you’re comparing options. If irritation doesn’t settle within a week or two of stopping, see your doctor or pharmacist.

The bottom line: glycolic and lactic acid do the same basic job. Glycolic is smaller, deeper and punchier; lactic is gentler and adds hydration. Start with the one that matches your skin, keep the strength modest, and treat sunscreen as part of the deal.