Glycolic Acid vs Lactic Acid, Honestly: Which One, How Often — and the Sun Warning on Both

Glycolic and lactic acid are the two exfoliating acids you are most likely to find in a toner, serum or peel. Most comparisons online say the same thing — glycolic is the strong one, lactic is the gentle one — and most of them are written by brands that sell one or both. So is this one: we make a product with each.

So this is the version with the sources attached: what each acid does, what the trials that compared them found, which one stings less, what the sun warning on both actually means, what the EU’s scientists advised, and exactly what is in ours. If you only want the routine, skip to how often to use them.

What glycolic and lactic acid do

Both are alpha hydroxy acids (AHAs): small, water-soluble acids that loosen the bonds holding dead cells together in the skin’s outer layer. The cells then shed more evenly, which is why skin feels smoother and can look brighter and more even. The two differ in size and in what else they do:

• Glycolic acid is the smallest AHA, with two carbon atoms. Its small size is the usual explanation for why it penetrates easily — and why it stings more easily.
• Lactic acid is one carbon atom larger, and it comes in two mirror-image forms. The natural form, L-lactic acid, was the one tested in the 22-week trial below. It also acts as a moisturiser: a 1996 comparison found that both forms moisturised better when partly or fully neutralised.

As acids they are almost equally strong, and that points to what matters more than the name on the bottle: the strength and the pH of the formula. AHAs exfoliate in their free-acid form, and the lower the pH, the more free acid there is — and the more a product stings. When the EU’s scientific committee on cosmetics of the time reviewed the evidence in 2000, it concluded that pH was the most important factor in how irritating an AHA product is, more important than its concentration. In one test, 13% glycolic acid at pH 4.4 was less irritating than 8% at pH 3.25.

Glycolic acid vs lactic acid: what the trials found

The best head-to-head evidence is a 1996 trial at Massachusetts General Hospital in Boston. Seventy-four women aged 40 to 70, with moderately severe sun damage to their facial skin, applied a cream with 8% glycolic acid, 8% L-lactic acid or no acid, twice a day for 22 weeks. On the face, 76% of the glycolic acid group and 71% of the lactic acid group improved by at least one grade of overall sun damage, against 40% of those using the plain cream. On the forearms, glycolic acid beat the plain cream on overall sun damage and sallowness, and lactic acid on overall sun damage, mottled pigmentation, sallowness and roughness. The researchers judged both creams modestly useful and well tolerated; one participant left the trial because of irritation.

Glycolic acid on its own: in a 1998 Australian trial, 75 volunteers applied a cream with 5% glycolic acid or a placebo cream to the face and neck every day for three months. Skin texture and discolouration improved significantly more with glycolic acid. Wrinkles showed a trend in its favour that was not statistically significant.

Lactic acid and strength: a 1996 study compared 5% and 12% lactic acid, applied twice a day for three months. Both strengths produced similar visible improvements and similar changes in the skin’s outer layer. Only 12% also changed the deeper layer, the dermis, making it firmer and thicker.

On comfort, the most direct comparison comes from the same year. A cosmetics consultant tested several AHAs at equal molar strength over six weeks. Glycolic acid and L-lactic acid were the most effective, with comparable improvements in hydration and fine lines — but lactic acid produced fewer complaints, and at the highest strengths tested it stung less than glycolic acid.

Which one stings less?

Lactic acid, usually. In the 1996 comparison it produced fewer complaints than glycolic acid, and the gap in stinging widened as the strength went up. Add its moisturising effect, and it is easy to see why lactic acid is so often recommended for dry or sensitive skin.

But gentler is not the same as gentle. Lactic acid is the acid dermatologists use to test for sensitive skin: in the classic lactic acid sting test, a 10% solution is dabbed into the crease beside the nose, and people whose skin stings are classed as sensitive. The EU committee’s 2000 review also reported a study in which even a 0.5% mixture of lactic and glycolic acid, applied once a day for six weeks, caused some irritation — mostly itching and stinging — in 26 of 100 people.

In practice: if your skin is dry or reacts easily, lactic acid is the better first choice, used less often and after a patch test. If most products sting your skin at the moment, neither acid is the answer yet — start with your skin barrier.

How often should you use glycolic or lactic acid?

The trials do not settle it. They tested low-strength creams applied once or twice a day, for months, in volunteers who were being monitored. They show that daily use of a 5–8% cream is tolerated by many people; they do not show that more often is better. Irritation tends to build with frequency, and the extra sun sensitivity lasts for as long as you keep using an AHA. A sensible way to start:

• Weeks 1–2: two evenings a week, not on consecutive nights.
• Then: add one evening at a time, every two weeks, as long as your skin stays comfortable.
• Stop increasing at the frequency your skin tolerates, or at the maximum on the label — whichever comes first.
• Leave-on peels are different. A stronger treatment that stays on overnight, such as our 10% lactic acid peel, is made for one or two nights a week, not every night.

Signs you are overdoing it: stinging that lingers, redness, tightness, flaking, or products that never used to sting starting to. Stop all acids until your skin settles; our skin barrier guide explains how.

Can you use glycolic acid every day?

Some people can. In the trials above, 5% and 8% glycolic acid creams were used once or twice a day for three to five months. But those were creams designed for daily use, and a stronger product, a lower pH or skin that already reacts easily changes the answer. Follow the directions on your product, and let your skin, not the calendar, decide.

How often to use a glycolic acid toner

An exfoliating toner is a leave-on acid, not a cleansing step, so the same rules apply: two evenings a week to start, on clean, dry skin, followed by moisturiser, and never on the same evening as a peel, a retinoid or another acid. Our Glycolic Acid Exfoliating Toner is designed for a few evenings a week.

How long does glycolic acid take to work?

Longer than most labels suggest. In the 1996 comparison, hydration and fine lines had improved within six weeks, but the trials that judged visible results did so after three months (5% glycolic acid) and 22 weeks (8% glycolic and lactic acid). Give any AHA eight to twelve weeks of steady use, with daily sunscreen, before deciding whether it works for you — and judge it in the same mirror and the same light.

The sun warning on both

This is the part most comparisons leave out. In studies commissioned by the US Food and Drug Administration, volunteers applied 10% glycolic acid at pH 3.5 to their backs six days a week for four weeks. Their skin became 18% more sensitive to reddening by UV, and its sensitivity to UV damage in skin cells doubled on average. One week after they stopped, the difference had gone.

A later study by FDA scientists found the same with 10% glycolic acid, while 2% salicylic acid, the usual acid in BHA products, did not increase UV sensitivity. Lower strengths are not automatically safe either: reviewing further data in 2004, the EU committee concluded that AHAs do increase UV damage to the skin, that glycolic acid at 2.5–4% (at pH 3.5) may do so too, and that the data could not identify a strength without any effect.

Most of that research used glycolic acid, but the warnings cover all AHAs. In 2005, the FDA issued guidance recommending that cosmetics containing AHAs carry this label:

Sunburn Alert: This product contains an alpha hydroxy acid (AHA) that may increase your skin’s sensitivity to the sun and particularly the possibility of sunburn. Use a sunscreen, wear protective clothing, and limit sun exposure while using this product and for a week afterwards.

Applying acids in the evening is sensible, but it is not enough on its own: in the FDA’s research, the extra sensitivity took about a week to disappear after the last application. What protects you is broad-spectrum sunscreen every morning while you use an AHA, and for a week after you stop.

Is there an EU limit on glycolic or lactic acid?

No legal one. EU cosmetics law does not restrict either acid, unlike kojic acid or retinol, which it caps. Instead, every cosmetic sold in the EU must pass a safety assessment before it goes on the market.

There is scientific advice, though. In 2000, the EU’s scientific committee on cosmetics at the time — a predecessor of today’s SCCS — said the data were not sufficient for a full safety assessment. As a precaution, until better data existed, it suggested glycolic acid at up to 4% with a pH of at least 3.8, and lactic acid at up to 2.5% with a pH of at least 5, with warnings to avoid the eyes and to protect skin from UV. In 2004 it reviewed new industry data, found it inadequate, and kept that advice. It has never been written into law.

In the US, the Cosmetic Ingredient Review, the industry-funded expert panel that assesses cosmetic ingredients, concluded that glycolic and lactic acid are safe in consumer products at up to 10%, provided the pH is at least 3.5 and the product either protects against sun sensitivity or tells you to use sun protection every day.

What that means for ours: both are stronger than the EU committee’s precautionary figures — the toner is 5% glycolic acid and the peel is 10% lactic acid — and within the US panel’s 10% ceiling. We would rather you read that here than discover it elsewhere, and it is one more reason to follow the directions: a few evenings a week for the toner, one or two nights a week for the peel, and sunscreen every morning.

Glycolic or lactic acid: how to choose

Blackheads and oily skin are a different case. Salicylic acid, a beta hydroxy acid (BHA), is oil-soluble, and it is the usual choice there. Our Purifying Toner contains a little salicylic acid, but alcohol is its second ingredient, so it suits oily skin rather than dry.

And if dark spots are your main concern, treat an exfoliating acid as a helper rather than the treatment. Both acids improved uneven, sun-related discolouration in the trials above; for fading dark spots themselves, see kojic acid vs alpha-arbutin.

What is in ours

We make one product with each acid. Here is exactly what they contain.

Glycolic Acid Exfoliating Toner

Our Glycolic Acid Exfoliating Toner contains 5% glycolic acid, the second ingredient after water. Potassium hydroxide partly neutralises it, which raises the pH; glycerin, betaine and sodium PCA help the surface hold water, and cranberry and lingonberry extracts are there in small amounts. There is no added fragrance, and a little alcohol comes last on the list. It is designed for a few evenings a week on clean, dry skin; follow it with moisturiser.

One honest caveat: the 1998 trial used an unneutralised 5% cream, and ours is a partly neutralised toner, so that trial’s results are a guide to what glycolic acid can do, not a promise for our product.

AHA Peeling Concentrate

Our AHA Peeling Concentrate is the stronger of the two: 10% lactic acid in a leave-on treatment that you apply in the evening and leave overnight, once or twice a week. It has nine ingredients and no fragrance — water, lactic acid, pentylene and butylene glycol, potassium hydroxide to adjust the pH, glycerin, sodium hyaluronate (a form of hyaluronic acid), and organic white tea and sage leaf extracts. Its label warns against direct sun after use.

10% is also the strength of the lactic acid sting test. The test uses plain lactic acid in water rather than a finished, pH-adjusted formula, but if your skin reacts easily, the comparison is worth knowing: patch-test first, start with one night a week, and never use it on the same night as the toner or a retinoid.

Neither product treats any skin condition. They are cosmetics that help skin feel smoother and look brighter and more even — and they only do that safely alongside daily sunscreen.

How to use them

1. Patch-test first: a small amount on the inner arm or behind the ear, once a day for a few days.
2. Evenings only, on clean, dry skin. Keep away from the eyes, and skip broken, irritated or sunburnt skin.
3. One acid per evening. Do not layer the toner and the peel, and keep acids and retinoids on different evenings.
4. Moisturiser next. A plain, fragrance-free one is the kindest choice after an acid — ours is the Sensitive Skin Moisturiser, Fragrance Free.
5. Sunscreen every morning, and for a week after you stop. Ours is the tinted SPF50 stick.
6. Build up slowly: two evenings a week to start, one more every two weeks if your skin stays comfortable.
7. Judge it at 8–12 weeks, not after a few days.
8. Stop if it stings, reddens or flakes and does not settle. Let your skin recover, then try again less often.

What to combine them with

• Niacinamide: fine in the same routine; introduce the two a fortnight apart. We covered the evidence in our niacinamide article.
• Retinol and retinoids: on different evenings, and start one at a time. If you are weighing retinol up, see retinol alternatives.
• Peptides and vitamin C: in the morning, rather than straight onto a fresh acid — see peptides explained.
• Scrubs and cleansing brushes: not on acid evenings. Chemical and physical exfoliation on the same evening is how barriers get damaged.

Where to start

If your skin is normal to oily, tolerates actives, and your concern is dullness or rough texture, start with the Glycolic Acid Exfoliating Toner on two evenings a week. If your skin is dry, lactic acid is the better match: the AHA Peeling Concentrate on one night a week, after a patch test. Either way, the product that decides how well it goes is the tinted SPF50 stick, every morning. Everything we make for exfoliation is in exfoliators.

And if your skin is already stinging, red or flaky, the best exfoliant is none at all for a couple of weeks. Our skin barrier guide explains what to do instead.