Glycolic acid is the smallest of the alpha hydroxy acids: two carbon atoms and a molecular weight of approximately 76 daltons. This size explains almost everything it does to the skin, for better and for worse. It penetrates faster and more deeply than any other AHA, and for the same reason, it is also the one most likely to cause irritation.
The question is not whether glycolic acid works. It does. The question is at what concentration, at what pH, and on which skin.
Article written by Elisa Avalle, founder of LeLang Skin Care and a dermocosmetics specialist. Cover image generated with the support of artificial intelligence.
What is glycolic acid
In INCI, it is listed as Glycolic Acid. It belongs to the family of alpha hydroxy acids, the water-soluble acids that also include lactic, mandelic, malic, tartaric, and citric acid. Originally, it was extracted from sugar cane; today, in almost all cosmetics, it is synthetic, ensuring consistent purity and the absence of potentially sensitizing plant residues.
Because it is water-soluble, it works on the surface and in the upper layers of the epidermis, not inside the pore: for that, you need a fat-soluble acid such as salicylic acid. This distinction matters when choosing a product, because the two do not solve the same problem.
How it works on the skin
Glycolic acid weakens the bonds that hold the cells of the stratum corneum together. The immediate result is faster shedding of dead cells on the surface: the skin appears smoother and more even after just a few applications, and light is reflected more evenly. It is the effect everyone notices first.
There is, however, a second effect that is slower and more interesting. With continued use, the acceleration of cellular turnover is accompanied by an increase in the dermis's hydrating component and greater collagen synthesis. This is not a result you achieve in two weeks: it is measured over months, and only if use is regular and the barrier is not destroyed along the way.
Percentages and pH: the two numbers that matter
The percentage alone tells you nothing. What determines an AHA’s activity is the proportion of free acid, which depends on the formula’s pH in relation to the acid’s dissociation constant, which is approximately 3.8 for glycolic acid. At pH 3.8, about half of the acid is in free, active form; at pH 4.5, the proportion drops to around 18%. In other words: a well-buffered 8% formula at pH 3.8 releases much more active acid than the same percentage at pH 4.5, and a poorly formulated 10% product can be both less effective and more irritating than a well-formulated 5% product.
As for Europe, an important clarification that almost no one makes is needed: for alpha hydroxy acids, there is no legal limit. Regulation (EC) 1223/2009 does not regulate them in an annex, and the relevant reference is the opinion issued at the time by the European Scientific Committee, which, as a precautionary measure, indicated that use of up to 4% with a pH no lower than 3.8 is safe for consumers, accompanied by a warning to protect against UV rays during treatment. This is a recommendation, not an obligation: more concentrated formulas on the European market are perfectly legal, provided the safety assessor assumes responsibility for them.
In the United States, the benchmark is different and more permissive: up to 10% with a pH no lower than 3.5 for products intended for consumers, and up to 30% with a pH above 3 for professional products, applied by trained personnel and rinsed off immediately. True medical peels work at concentrations between 20% and 70% and much lower pH levels, and have nothing to do with a cosmetic product. When an online product promises 50% glycolic acid for home use, the problem is not effectiveness: it’s that no one is managing the risk.
Glycolic, lactic, mandelic, salicylic acid: which one to choose
| Acid | Molecule | Where it works | Suitable for |
|---|---|---|---|
| Glycolic acid | 76 Da, the smallest | Surface and upper layers, rapid penetration | Thick, dull skin with marks and spots, tolerant |
| Lactic acid | 90 Da | Surface, retains water | Dry or dehydrated skin |
| Mandelic acid | 152 Da, the largest among the AHAs | Surface, slow and gradual penetration | Reactive, acne-prone skin, darker skin tones |
| Salicylic acid | Oil-soluble | Inside the pore, in sebum | Oily skin, blackheads, clogged pores |
You need to distinguish between potency and suitability. Glycolic acid is the most potent in the family, but the most potent option is not the best for everyone: on skin that reddens easily, it is exactly the wrong choice. If you’re interested in the full overview of acids, you’ll find it in the guide to AHA, BHA, and PHA.
How to use it without damaging the skin barrier
- Apply it in the evening, to dry skin, after cleansing and before creams.
- Start with twice a week and increase only if your skin remains calm. There is no reason to use it every evening.
- Do not combine it with retinol in the same application: alternate evenings.
- The morning after, sun protection is not a recommendation; it is part of the treatment. AHAs increase the skin's sensitivity to UV rays, which is why the warning is mandatory on labels in the United States.
- If persistent burning, flaking, or stinging lasting beyond the first few minutes occurs, stop using it. Your skin is not simply getting used to it.
When it is best to avoid it
On skin with couperose or rosacea, on sensitive and reactive skin, or on a skin barrier already compromised by harsh cleansers or overly frequent exfoliation. It should also be avoided immediately after sun exposure, after waxing, or in the days leading up to a seaside holiday.
Sun spots are a separate matter: glycolic acid helps even out the complexion because it speeds up cell turnover, but if used without sun protection it can worsen the situation instead of improving it, especially on darker skin tones and with melasma. Exfoliation without sun protection is wasted effort, when it is not counterproductive.
Why we use mandelic acid in our formulas
Our chemical exfoliant uses mandelic acid and an enzyme component, not glycolic acid. This choice is linked to the channel: in pharmacies and para-pharmacies, the advice must also work for skin that does not tolerate rapid exfoliation, and mandelic acid, with a molecule twice the size of glycolic acid, penetrates more slowly and leaves a wider safety margin. Those with thick, tolerant skin can use glycolic acid with excellent results; those who redden easily, much less so.
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Frequently asked questions
Does glycolic acid lighten dark spots?
It does not act on melanin like a depigmenting active does: it speeds up cell turnover and therefore removes cells that are already pigmented. It works on superficial, recent spots; for deeper hyperpigmentation, something else is needed, along with daily sun protection.
How often can it be used?
Twice a week is a sensible frequency to start with. Three times is fine for skin that's used to it and is thicker. Beyond that, you enter territory where the benefits stop and barrier damage begins.
Does it have to sting to work?
No. Mild tingling in the first few seconds is normal; a burning sensation that persists is not. The idea that a cosmetic product has to hurt to be effective has no basis: irritation isn't a sign that it's working; it's a sign that the formula isn't suitable for that skin.
Sources for this article
- Regulation (EC) No. 1223/2009 on cosmetic products, which does not set concentration limits for alpha hydroxy acids.
- Opinion of the European Scientific Committee on Cosmetic Products and Non-Food Products Intended for Consumers (SCCNFP) on alpha hydroxy acids: as a precautionary measure, glycolic acid up to 4% with pH ≥ 3.8 and lactic acid up to 2.5% with pH ≥ 5, with warnings about eye contact and protection from UV rays.
- Cosmetic Ingredient Review (CIR), safety assessment of alpha hydroxy acids: up to 10% with pH ≥ 3.5 in consumer products, and up to 30% with pH ≥ 3 in professional products intended for brief contact followed by rinsing.
- Guidance from the U.S. Food and Drug Administration on alpha hydroxy acids and increased skin sensitivity to UV radiation, with a recommended warning on the label.
- Cosmetic dermatology literature on glycolic acid formulation: the relationship between dissociation constant, pH, free acid proportion, penetration, and irritation; clinical studies on stratum corneum turnover, dermal hydration, and collagen synthesis with prolonged topical use.
If you have to choose just one acid and your skin isn't used to them, don't start with the strongest one. Start with the one you can use consistently for six months: consistency produces results, not the percentage printed on the label.
