Ingredient Beauty BenefitsAcid · Glycolic Acid
Glycolic acid: the best evidence is from 1996, and the concentration was 8%
In a 22-week, 74-person, vehicle-controlled double-blind trial, **76% of people using 8% glycolic acid cream improved by at least one grade of photodamage** (vehicle: 40%). Thirty years on, we found nothing better for this ingredient. And the same molecule **did not protect the skin barrier.**
Last checked Sep 17, 2026
- In pregnancy
- Not established
- Irritation
- High
- Daytime use
- ! Sunscreen required
Not a measurement — reported irritation grouped into five steps. It varies between people. This ingredient makes skin more sensitive to UV.
Glycolic acid is the smallest alpha hydroxy acid (AHA). Its basic action is loosening the bonds between corneocytes so they shed.
For this ingredient, concentration and pH matter far more than the name. A 0.5% lotion and a clinic peel are not the same object even though both say 'contains AHA'. Fortunately Korea has a rule that lets you read this off the label. That is the last item below.
One thing first: this page is about leave-on and rinse-off cosmetics. High-strength peels performed in a clinic are a procedure, with different evidence and different risks.
Contents
What is established
The same ingredient at a different strength, in a different formula, is a different product. These are the conditions the studies used.
8% cream, 22 weeks, 76% against 40%
A single-centre, 22-week, double-blind, vehicle-controlled randomised trial run at Massachusetts General Hospital in 1996. Seventy-four women aged 40 to 70 with moderately severe facial photodamage took part.
They applied 8% glycolic acid, 8% L-lactic acid, or vehicle cream twice daily to the face and forearms.
The proportion improving by at least one grade in overall photodamage severity (0–9 scale) was 76% for glycolic acid, 71% for lactic acid and 40% for vehicle — a significant difference (P < .05). On the forearms at week 22, glycolic acid beat vehicle on overall severity and on sallowness.
It is more accurate to quote the authors' own wording for the conclusion: 'modestly useful'. Seeing that 40% of the vehicle group also improved gives you a sense of how much the acid itself contributed.
This trial also did not report pH — even though in an acid, pH matters as much as concentration. That is why we leave the field marked as unreported rather than inventing a number.
- Randomised controlled trial in people · 74 participants · 22 weeks · Compared with vehicle (same formula minus the active) · Funding not declared · Endpoint scores from blinded assessors PMID 8651713
A 15% cream produced changes visible on biopsy
The 2003 study had 65 postmenopausal women apply 0.01% estradiol cream, 15% glycolic acid cream, or the combination to one side of the face and vehicle cream to the other, for six months. A 2 mm punch biopsy was taken from the hairline of each participant.
All groups showed statistically significant improvement in markers of ageing — rete peg pattern and epidermal thickness. Epidermal thickness rose 38% in the combination group (P = .000181), against 23% for estradiol alone. A cumulative effect was seen but did not reach significance (P = .1).
Biopsy is the firmest kind of endpoint this ingredient has. It is not what someone felt, nor a grader's score; it is tissue.
That said, the trial was designed to look at estradiol, and the abstract does not give separate figures for the glycolic acid arm. So we do not write a sentence of the form '15% raised thickness by X%'.
- Randomised controlled trial in people · 65 participants · 26 weeks · Compared with vehicle (same formula minus the active) · Funding not declared · Endpoint biopsy findings PMID 12839261
The AHAs that protected the barrier were the other ones
This may be the most useful item on the page.
The 1997 trial marked six test sites on the arms and forearms of eleven healthy people and rotated them randomly: four AHAs at 8% (glycolic, lactic, tartaric, gluconolactone), vehicle only, and an untreated control. Products went on twice a day for four weeks.
At week 4 the sites were challenged with sodium lauryl sulphate, and transepidermal water loss and erythema were measured.
The results split. The gluconolactone and tartaric acid sites had significantly lower water loss after the challenge than vehicle and than untreated skin (P < .01) — and significantly lower than glycolic acid (P < .05). Erythema followed the same pattern.
The authors conclude that AHAs can modulate barrier function, but the effect is not equal for all AHAs, being more marked in the molecules with antioxidant properties.
'AHAs are good for the barrier' does not hold for glycolic acid. This is what happens when a result belonging to a class is transferred onto one member of it. Eleven people cannot settle the question, but a signal pointing the other way deserves to be written down.
- Randomised controlled trial in people · 11 participants · 4 weeks · Compared with vehicle (same formula minus the active) · Funding not declared · Endpoint instrument measurement PMID 9470910
The warning text on the label tells you the concentration band
Korea fixes by notice the wording that must appear on AHA-containing cosmetics: the Regulation on Precautions for Use and Labelling of Allergenic Fragrance Ingredients, Annex 1, item 2(b)(9).
Above 0.5%, these must appear:
- Use with sunscreen, as it may increase the skin's sensitivity to sunlight (rinse-off and hair products excepted)
- Test on a small area first to check for skin reaction
Above 10%, or at a pH below 3.5, one more line is added:
- Consult a specialist, as the high AHA content carries a risk of adverse effects
What this rule gives a reader is a way to read concentration. Even when a company does not disclose the percentage, the presence of that third line means the product is either above 10% or below pH 3.5. You cannot tell which — but both are conditions that make it act more strongly.
A note worth adding: many articles still cite this as Annex 3 of the Cosmetics Act Enforcement Rule. That annex was amended in 2022 and now holds only the general precautions. The ingredient-specific wording lives in this notice. We opened both original texts to check.
How to use it
- Do not use it without sunscreen. The notice mandates that line for a reason: thinned stratum corneum plus sunlight comes back as pigmentation.
- Start low and go up only if skin tolerates it. The trials above used 8% and 15%, but those were screened, supervised conditions.
- Two or three nights a week is a common starting pattern. Nothing in these trials says daily is required — the 1996 trial was twice daily, but under 22 weeks of managed conditions.
- Layering it with retinol on the same night stacks the irritation. Alternating nights is the easier path.
Cautions
- This is an irritating ingredient. Stinging, redness and flaking are common. One participant in the 1996 trial withdrew early because of irritation.
- The lower the pH, the harder it works. That is why the same percentage behaves differently across products.
- Do not use on broken skin or recently treated areas.
- We found no established data on use during pregnancy.
- High-strength peels performed in a clinic are outside the scope of this page: different concentration band, different risks.
With a new ingredient, apply a small amount somewhere inconspicuous, such as the inner arm, for two days first. Stop if it reddens or stings.
Related skin concerns
What this article relies on
- PubMed — 미국 국립의학도서관 문헌 데이터베이스National Library of Medicine · Checked on 2026-09-16
- 화장품 사용할 때의 주의사항 및 알레르기 유발성분 표시에 관한 규정 [별표 1] 화장품의 유형과 유형별·함유 성분별 사용할 때의 주의사항 표시 문구 — 식품의약품안전처고시 제2026-56호식품의약품안전처 · 시행 2026-08-05 · Checked on 2026-09-17