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Glycolic acid

Also known as: glycolic acid, AHA, alpha-hydroxy acid, hydroxyacetic acid

Glycolic acid is the smallest alpha-hydroxy acid, used at roughly 3–10% in cosmetic products and 20–70% as a professional peel agent. It does not self-neutralise: the reaction continues until an alkaline neutraliser stops it.

Evidence status

Moderate

Unusually uneven across indications. Photoageing is the strongest: a 22-week double-blind vehicle-controlled RCT in 74 women (76% vs 40% improvement) plus supportive histology. Melasma has genuine RCT evidence but only active-comparator, small (10 per arm), single-centre and without an untreated control. Acne is the weakest link — Cochrane found a single 40-participant comparison and no vehicle-controlled trial. Skin-of-colour data are thinnest of all: one 1997 pilot with 16 completers. Sun sensitisation is directly demonstrated by a randomised double-blind trial (10% leave-on, not a peel) and is reversible within a week. The professional peel parameters everyone quotes — 20-70%, three-minute exposure, bicarbonate neutralisation — rest on expert-review and guideline convention, not on dose-ranging trials. And the regulatory picture is the outlier: contrary to almost universal industry belief, glycolic acid carries no statutory concentration or pH limit in either the EU or GB.

What it is#

Glycolic acid is the smallest alpha-hydroxy acid — molecular weight 76.05 g/mol, and strongly water-loving rather than oil-loving.[13, 10]

It is the workhorse superficial peel agent in professional practice and one of the most-used actives in cosmetic skincare. It is also surrounded by more confidently-repeated misinformation than almost anything else in this encyclopedia, starting with its legal status.

The limit that isn't#

Ask any group of practitioners what the legal limit on glycolic acid is and you will hear “4% and pH 3.5 for retail, 70% for professional”. Both halves are wrong.

Glycolic acid appears nowhere in Annex II or Annex III of the EU Cosmetics Regulation as consolidated to 1 May 2026. There is no legally binding maximum concentration and no minimum pH, for consumer or professional products.[2]

The GB assimilated version of Annex III is the same — no entry, so no statutory concentration or pH limit applies in Great Britain either.[3]

Every apparent hit in both texts turns out to be thioglycolic acid, an unrelated substance used in perming and depilatories.

So where do the numbers come from?

The 4% figure is a precautionary recommendationfrom the EU’s scientific committee in 2000, reaffirmed in 2004. It specified pH at or above 3.8 — not 3.5 — and it was never written into the Annexes.[1, 2]

The 10% at pH 3.5 figure, and the professional figure, come from the US Cosmetic Ingredient Review panel. They are non-binding even in the United States, and the salon figure is 30% at pH 3.0 — not 70%.[12]

No ingredient-specific UK cap exists — but note what we did not search. We did not complete a jurisdiction-by-jurisdiction check of statutory qualification, premises and local-authority licensing requirements across the four nations, and Scotland now has a 2026 framework Act with detail still to come in regulations. Treat the absence of a named mandatory qualification as unsearched rather than settled.[no source found]

What actually governs safety is the general obligation: a Responsible Person must hold a Cosmetic Product Safety Report and the product must be safe in normal or reasonably foreseeable use. That is a real constraint — it is just not a number, and quoting a number as law is factually wrong.

Size and penetration#

The standard explanation is that glycolic acid penetrates best because it is the smallest AHA. Worth being precise about what is actually established there.

The expert review most often cited describes glycolic acid, in a single breath, as the smallest and simplest AHA, highly hydrophilic, and the AHA with the greatest bioavailability. It does not state that the first two cause the third — that causal link is the conventional explanation practitioners attach to it.[10]

And the bioavailability ranking is an expert assertion rather than a finding from a head-to-head human penetration study. We looked for such a comparison and did not find one.[10]

We searched and found no head-to-head human comparison of AHA skin penetration at all.[no source found] The molecular-weight explanation is chemically plausible. It has not been demonstrated comparatively in people, and a review asserting a ranking cannot itself establish that no such study exists — which is what this page previously implied.

Concentrations#

  • Cosmetic and over-the-counter:typically 3–10% — though a US market survey of about 100 AHA products found total AHA concentrations spanning 0.01% to 67%.[10, 12]
  • Professional:roughly 20–70%.
  • And contact time decides the rest.A widely used classification puts 50–70% for two to five minutes in the superficial category and 70% for three to fifteen minutes in the medium-depth one. The same number, on either side of the line.[10, 15]

This page previously said “at 70% you are performing a medium-depth peel, whatever the menu calls it”. That turned one review’s convention into a deterministic scope rule, and it contradicted our own chemical-peel entry. What is true is that intensity is set by concentration, vehicle, quantity applied and technique together — so describe your formulation, your protocol and your achieved endpoint, and treat 70% as a strength that can reach medium depth rather than one that always does.

Neutralisation#

The defining practical property of glycolic acid, and the one that makes it more dangerous than its reputation suggests.

Glycolic acid does not reliably self-limit. Keratocoagulation continues for as long as the acid remains on the skin, so exposure has to be ended at the planned time or the clinical endpoint.[10, 11, 14]

What it does not require is one alkaline recipe.This page said the reaction “has to be actively stopped with an alkaline neutraliser”, which is a safety-critical absolute the evidence does not support. A randomised, double-blind, split-face trial in 126 adults with phototypes IV–V found water an effective substitutefor sodium bicarbonate after a 35% glycolic peel — with significantly lesspain immediately after neutralisation, and no meaningful difference in erythema or pruritus. The stopping method is formulation- and protocol-specific. Follow the manufacturer’s validated instructions.

The familiar three-minute, 10–15% sodium bicarbonate protocol — and the instruction to stop immediately, regardless of the timer, if erythema or epidermolysis appears — comes from a 2008 Indian professional-association consensuswritten for physician-led dermatology practice. It is not a UK rule, a dose-ranging trial, or every product’s instructions for use. The stop-on-epidermolysis half of it is sound and worth keeping; the recipe belongs to that document. Epidermolysis looks like greyish-white epidermis or small blisters.[11]

Warn the client that neutralising stings. Acid-base neutralisation is exothermic, so there is a transient surge of warmth, burning or stinging at exactly the moment the client expects relief. An unwarned client reads that surge as the peel going wrong.[10]

A disagreement worth knowing about

Our chemical peel entry reports the International Peeling Society’s position that only glycolic and pyruvic acid require neutralisation.

The expert review underpinning this entry says something broader: that glycolic, lactic and pyruvic acid all need an alkaline neutraliser, and that salicylic acid is the exception, self-neutralised by the skin’s own endogenous lipoproteins. It contrasts salicylic with “glycolic acid and the other AHAs”, which places mandelic acid on the neutralise side by implication rather than by direct statement.[10]

Two expert sources genuinely disagree about lactic acid, and we are not going to resolve it with a house rule. The International Peeling Society says only glycolic and pyruvic need neutralising; Soleymani says lactic does too. On mandelic acid, Soleymani gives no instruction at all — it lands on the neutralise side only by inference from a class-level contrast.

So do not infer a stopping protocol from the acid family. Follow the instructions for the exact product in front of you, and never carry a salicylic acid habit across to an AHA. This page previously resolved the disagreement by telling you to “assume an AHA needs neutralising”; that is a reasonable instinct and it is our rule rather than a finding, which is not how this entry should be settling a conflict between sources.

Endpoints#

For a superficial glycolic peel the endpoint is the product-specific time and clinical endpoint — not a frost level. Glycolic-specific guidance is explicit that whitening is a stop signal: “If frosting is observed in any particular area before the set time or end-point, it is important to neutralize the peel immediately.” Erythema, grey-white epidermolysis and blistering are read the same way.[10, 15]

This page previously offered Level-I frosting as the target and seven to ten days as the healing time. Both were borrowed from a generic superficial-peel scale rather than from anything glycolic-specific, and presenting a stop signal as something to aim for is the wrong way round.

Frosting is a depth signal on a TCA peel and an overshoot alarm on a superficial glycolic peel. Same observation, opposite meaning, depending on the agent in your hand.

Sun sensitivity#

Real, measured, and more time-limited than the warnings suggest.

In a randomised, double-blind trial, 29 subjects had their backs treated once daily, six days a week for four weeks, with 10% glycolic acid at pH 3.5 or placebo. In the 16 assessed for it, glycolic acid produced enhanced UV sensitivity — more sunburn cells and lowered minimal erythema doses. In a separate group of 13 assessed for DNA damage, pyrimidine dimers were elevated but not significantly.[4]

It is not permanent. No difference in sunburn cells or minimal erythema dose remained one week after stopping.[4]

The EU’s scientific committee reached the same direction: AHA application does increase UV damage, a glycolic formulation at pH 3.5 in the 2.5–4.0% range may lower the MED and raise sunburn-cell counts, and no no-effect level could be defined.[1]

With a caveat the committee itself supplied. It judged the underlying MED dataset uninterpretable because the standard deviations were so large, and reaffirmed its limits only because the industry data submitted were inadequate. The position rests on precaution as much as on measurement.[1]

On cancer, the only photocarcinogenesis evidence located is a 12-month hairless-mouse study in which glycolic acid at 4% or 10% plus SPF15 did not increase tumour formation. It is animal work with no sunscreen-only control, so it neither establishes nor rules out a human risk — and should not be offered to clients as reassurance.[1, 16]

What it treats#

Photoageing — the strongest case

Genuine vehicle-controlled evidence: a 22-week double-blind trial enrolled74 women aged 40–70 with moderately severe facial photodamage, and 67 completed. At least one grade of improvement on a 0–9 photodamage scale was reported in 76% on 8% glycolic acid, 71% on 8% L-lactic acid and 40%on vehicle. No confidence interval was reported, and the authors’ own word for the effect is “modestly useful”.[8]

Two things this page used to lose. The percentages were quoted against the enrolment figure rather than the completers, and the lactic-acid arm — which did nearly as well — disappeared. And it was an 8% twice-daily leave-on cream, not a peel: nothing here transfers to a 35% or 70% professional treatment.

The histological support is weaker than its citation count suggests, and it is not glycolic-specific. A 17-person split-forearm pilot allocated five participants to glycolic acid, five to lactic and seven to citric, then pooledthe results across all three and biopsied eight people. It reported roughly a 25% increase in skin thickness with increased dermal collagen density. It cannot isolate a glycolic-specific effect — and two authors were the patent-holders and company founders behind AHA skincare.[9]

Melasma — decent, with limits

In a 30-person active-comparator trial, every arm received four 35% full-face glycolic peels at 15-day intervals, ten patients per arm. MASI fell within each group. Adding a 10% or 20% TCA spot peel in the same sitting produced no statistically detected between-group benefit, and the glycolic-only group had the fewest reported side effects — with the authors saying the combinations may increase them.[6]

Read both halves carefully. Within-group change with no untreated or vehicle arm cannot tell you how much the peel itself produced. And a null superiority result with ten per arm, no non-inferiority margin and no power calculation establishes neither equivalence nor the reason it was null — this page previously asserted the second, which is a guess dressed as an explanation.[6]

Acne — close to empty

Cochrane included fiveglycolic-acid peel studies using 20–70% — so the literature is not as empty as this page once said. What is thin is the usable output. Only one40-participant active-comparator trial contributed participant-global-assessment data, showing no difference against a salicylic-mandelic peel (RR 1.06, 95% CI 0.88–1.26) at low certainty; two comparisons totalling 84 participants contributed only withdrawal data. Everything else was heterogeneous, incompletely reported or unusable. No placebo- or vehicle-controlled glycolic acid comparison appears anywhere in the review’s results.[5]

That is not evidence of no effect. It is an absence of evidence, and it means glycolic peels cannot honestly be sold as evidence-based acne care.

Darker skin#

The thinnest evidence in the entry, on the question where the stakes are highest.

In post-inflammatory hyperpigmentation in Fitzpatrick IV–VI skin, adding six serial glycolic peels to a hydroquinone-plus-tretinoin regimen produced what the authors describe only as a trend toward greater improvement in 16 completers, with no supporting statistics reported. The peel group also developed lightening of the surrounding normal skin.[7]

That second finding matters as much as the first. The risk to consent for is not only post-inflammatory hyperpigmentation but uneven lightening of untreated skin.

The most-cited guidance is a 2008 Indian expert consensuswritten for physician-led dermatology practice. It holds superficial peels safe in that setting, medium-depth peels as demanding great caution in dark-skinned patients, and deep peels as not recommended, with 2–4 weeks of priming using sunscreen, hydroquinone and tretinoin called essential.[11]

Be careful what you read into it.This page described it as written for a “predominantly Fitzpatrick III–V population”. The document reports no study cohort and no Fitzpatrick denominator at all— that distribution was inferred from geography, which is not a measurement. It is a consensus for another country’s physicians, not a UK guideline and not a V–VI trial.

And the priming step is not available to a non-prescriber here. In Great Britain hydroquinone is prohibited as a cosmetic ingredient (Annex II 1339) and tretinoin is prohibited outright (Annex II 375); any medicinal route belongs to an appropriately authorised prescriber working within competence. Northern Ireland follows a separate regime and should be checked rather than assumed to match.

In professional practice#

  • Stop quoting an ingredient-specific legal limit that does not exist— and stop at that. The accurate answer is that the Annexes set no maximum concentration and no minimum pH for glycolic acid; it is not that there are no rules. Safety still sits with the Responsible Person’s assessment, with the product meeting the cosmetic definition, and with your protocol.
  • Know how your product is meant to be stopped, before the acid goes on.The trigger to stop immediately is erythema or epidermolysis, not the timer — but the method is the manufacturer’s, not a universal bicarbonate recipe.
  • Warn about the sting on neutralisation. It is exothermic and it is expected.
  • Treat frosting as an alarm, not a target, on a superficial glycolic peel.
  • Do not carry salicylic habits across. Salicylic self-neutralises. Whether a given AHA needs an alkaline neutraliser, water, or timed removal is a question for that product's instructions — the sources disagree even about lactic acid.
  • Know that 70% can be either. Superficial at two to five minutes, medium-depth at three to fifteen — so the consent conversation follows your protocol, not the percentage.
  • Sun protection is a treatment component. The only measured washout is a week after stopping a 10% leave-on cream on back skin; nobody has measured it after a peel.
  • Be honest about acne. Five studies were included and only one produced a usable response comparison.

What remains uncertain#

  • The regulatory finding rests on programmatic search of consolidated texts, which carry a disclaimer that they have no legal effect and will not reflect a measure adopted but not yet consolidated. Worth re-checking before anyone relies on it commercially.
  • No trial establishes an optimal exposure time, neutralisation timing or session interval. The three-minute convention is consensus, not a dose-ranging result.
  • No RCT of glycolic acid conducted specifically in Fitzpatrick V–VI, and no published complication rates broken down by phototype.
  • No placebo- or vehicle-controlled trial of a glycolic peel for any indication — every peel trial located used an active comparator.
  • Because there is no statutory limit, professional product strength is effectively self-declared. Labelled percentage, actual pH and free acid content need not correspond, and no independent verification of marketed professional solutions was located.

Common misconceptions#

“The EU and UK cap it at 4% and pH 3.5.”

Not law. Neither text contains a glycolic acid entry. The 4% figure is a non-binding recommendation, and it said pH ≥ 3.8.[1, 2]

“Professional glycolic is legally capped at 70%.”

There is no ingredient-specific cap in the Annexes — which is not the same as there being no rules, and 70% is convention rather than law. The only quantified professional figure located is the US panel’s 30% at pH 3.0, and that panel is an industry self-regulatory body funded by the Personal Care Products Council; the FDA quotes its conclusions without adopting them, and they carry no UK weight.[12]

“AHAs make skin permanently sun-sensitive.”

In that trial — 29 Caucasian participants, 10% leave-on, on the back, for four weeks — the excess sunburn-cell induction and the minimal-erythema-dose difference were no longer detected a week after stopping, in the 16-person endpoint group. That is a washout for that model. It says nothing about photoprotection after a professional peel, which was never studied.[4]

“Glycolic peels are an established acne treatment.”

Cochrane found one 40-participant active-comparator study at low quality and no vehicle-controlled trial at all.[5]

“The classic study proves your 8% serum builds collagen.”

That study used 25% AHA on forearm skin, pooled three different acids, was explicitly a pilot, and two authors were the patent-holders behind AHA skincare.[9]

Frequently asked questions#

What strength am I legally allowed to use?

The Cosmetics Regulation sets no limit for glycolic acid at all. What constrains you in practice is your insurer, your qualification, your supplier’s instructions and the product’s safety assessment — none of which is the same as a statutory cap.[3]

How long do I leave it on?

Convention is about three minutes, but the real answer is: until the timer or the skin tells you to stop, whichever comes first. Erythema or epidermolysis means neutralise now.[11]

My client says it burned more when I neutralised it.

That is expected — the reaction is exothermic. Warn them beforehand and it stops being alarming.[10]

How long do they need to be careful in the sun?

Honestly, nobody has measured it after a peel. What was measured is a 10% leave-on cream on the backs of 29 Caucasian participants: the excess sunburn-cell induction and the MED difference were no longer detected a week after stopping. Treating that as a post-peel window is a precaution borrowed from a different model, not a result — and this page previously gave it as an instruction.[4]

Is it safe on Fitzpatrick V–VI?

Nobody has established that. The consensus that calls superficial peels safe was written for physician-led Indian practice and reports no Fitzpatrick denominator, so it cannot be read as V–VI evidence.[11]The one darker-skin study enrolled phototypes IV–VI, had 16 completers, reported only a trendwith no supporting statistics, gave no V–VI subgroup — and found lightening of surrounding normal skin as well as the intended effect.[7]

References#

Each source is graded by evidence tier. Tier 4 material (manufacturer documents, expert consensus, practitioner experience) is useful for protocol and context, and is never presented as equivalent to independent clinical evidence.

  1. Scientific Committee on Cosmetic Products and Non-Food Products Intended for Consumers (SCCNFP). Updated Position Paper Concerning Consumer Safety of Alpha-Hydroxy Acids. SCCNFP/0799/04. Adopted at the 28th plenary meeting, 25 May 2004.Tier 1Supports: Full text read. Reaffirms the earlier SCCNFP/0370/00 position (28 June 2000) that, on the precautionary principle, 'glycolic acid may be used safely at a level of up to 4 % and a pH >= 3.8' and 'lactic acid up to a maximum level of 2.5 % and a pH >= 5.0'. Note the glycolic pH figure is 3.8, NOT 3.5. On UV: 'From the results obtained, it can be deduced that AHA application does increase UV damage to the skin. A skin treatment with a glycolic formulation at pH 3.5, in the concentration range 2.5-4.0 %, may induce a decrease in MED and/or an increase in SBC production.' and 'It was not possible from the data available to define a NOEL.' Underlying MED data (mJ/cm2, mean +/- SD): glycolic 2.5% 66.4+/-13.0; 4.0% 62.6+/-11.8; 6.0% 58.5+/-12.7; vehicle 66.8+/-15.5; untreated skin control 73.1+/-17.0 — on which the Committee itself states 'Because the standard deviations (SD) of these data are so large, nothing can be concluded.' Final conclusion: 'The SCCNFP maintains its previous opinion (SCCNFP/0370/00, 28 June 2000) because of the inadequate nature of the data submitted for evaluation.' Also summarises a 12-month photocarcinogenesis study in Crl:SKH1-hrBR hairless mice (144/sex), glycolic acid 4% or 10% at pH 3.5 plus SPF15 sunscreen, solar-simulated UVR 5 days/week for 40 weeks then 12 weeks observation: tumour potency ratios 0.64 for all glycolic+sunscreen groups vs 1 (untreated control) and 2 (UVR-only control); 'it may be suggested that topical administration of glycolic acid together with sunscreen formulations did not enhance photocarcinogenesis. However, no control using the sunscreen formulation alone was included in the experiment.' SCOPE LIMITS: this is an advisory opinion, not law; it addresses consumer leave-on cosmetics only; it contains NO professional/salon concentration or pH figure anywhere in the document; the mouse photocarcinogenesis data are animal data.Funding / interest: EU Commission scientific committee. Notably, all four TEWL/barrier and UV-sensitivity datasets it assesses were supplied by the cosmetics industry ('Submission IV presented by the Cosmetic Industry'; CTFA), and the Committee repeatedly rejects them as inadequate ('the study is not acceptable'; 'These experimental data are insufficient for adequate statistical evaluation').
  2. Regulation (EC) No 1223/2009 of the European Parliament and of the Council of 30 November 2009 on cosmetic products (recast), consolidated text CELEX 02009R1223-20260501, in force from 1 May 2026. Annexes II and III.Tier 1Supports: Full consolidated HTML (3.85 MB) downloaded and searched programmatically. Glycolic acid is ABSENT from the entire Regulation: zero occurrences of CAS 79-14-1, zero of EINECS 201-180-5, zero of 'hydroxyacetic', zero of 'alpha-hydroxy'. The only five matches for the string 'glycolic acid' are all THIOGLYCOLIC acid (Annex III entries 2a and 2b, CAS 68-11-1, hair waving/straightening, depilatories, eyelash waving) — a chemically unrelated substance. 'Lactic acid' also returns zero occurrences. For contrast, 'salicylic acid' returns 6 occurrences, confirming the search method finds substances that ARE listed. CONCLUSION: as at the 1 May 2026 consolidation there is no entry for glycolic acid in Annex II (prohibited) or Annex III (restricted), and therefore no legally binding EU maximum concentration and no legally binding minimum pH for glycolic acid in cosmetic products, consumer or professional. SCOPE LIMITS: EUR-Lex states consolidated texts are 'meant purely as a documentation tool and ha[ve] no legal effect'; this establishes the absence of a listed restriction at that date and says nothing about any proposal not yet adopted.
  3. Regulation (EC) No 1223/2009 on cosmetic products, Annex III, as it forms part of assimilated law in Great Britain. legislation.gov.uk, page metadata DC.Date.Modified 17 July 2026.Tier 1Supports: Annex III of the GB assimilated version downloaded and searched programmatically. Same result as the EU text: zero occurrences of CAS 79-14-1, zero of 'hydroxyacetic', zero of 'lactic acid'; all five 'glycolic acid' string matches are thioglycolic acid (entries 2a and 2b). There is therefore no GB-assimilated restricted-substance entry for glycolic acid, and no statutory concentration or pH limit for it in cosmetic products sold in Great Britain. SCOPE LIMITS: Annex III only was retrieved; Northern Ireland follows the EU regime, which the separate EU check (source 2) shows is the same in substance for this ingredient.
  4. Kaidbey K, Sutherland B, Bennett P, Wamer WG, Barton C, Dennis D, Kornhauser A. Topical glycolic acid enhances photodamage by ultraviolet light. Photodermatol Photoimmunol Photomed. 2003 Feb;19(1):21-7. PMID 12713551.Tier 2Supports: Randomised, double-blinded study. Backs of 29 Caucasian subjects treated once daily, 6 days per week, for 4 weeks with either 10% glycolic acid at pH 3.5 or placebo. At 4 weeks, treated sites exposed to 1.5 MED of UV. Sunburn cells and MED measured in the first group (n=16); cyclobutyl pyrimidine dimers in DNA in the second (n=13). Results verbatim: 'Glycolic acid caused enhanced sensitivity to UV light measured as increased SBC induction and lowered MEDs. Cyclobutyl pyrimidine dimers were elevated but not to a statistically significant level. No differences in SBCs or MEDs were evident after a week of discontinued treatments.' Conclusion verbatim: 'Short-term application of 10% glycolic acid sensitizes the skin to the damaging effects of UV light. This photosensitivity is reversed within a week of terminating treatments.' SCOPE LIMITS: Caucasian subjects only — no phototype IV-VI data; back skin, not face; a 10% leave-on product at pH 3.5, NOT a professional peel, so it does not quantify post-peel photosensitivity; abstract reports no effect sizes or p-values for the positive findings; DNA-damage endpoint (CPDs) did not reach significance.Funding / interest: No funding statement retrievable from the PubMed record. Lead author affiliated with Ivy Laboratories, Philadelphia, a contract clinical-testing laboratory.
  5. Liu H, Yu H, Xia J, Liu L, Liu GJ, Sang H, Peinemann F. Topical azelaic acid, salicylic acid, nicotinamide, sulphur, zinc and fruit acid (alpha-hydroxy acid) for acne. Cochrane Database Syst Rev. 2020 May 1;5(5):CD011368. PMID 32356369, PMCID PMC7193765.Tier 1Supports: Cochrane systematic review, databases searched to May 2019, 49 RCTs and 3,880 participants, mostly mild-to-moderate acne, aged 12-30 (range 10-45), mostly female. The ENTIRE alpha-hydroxy acid result set is: 'There may be no difference in PGA when comparing glycolic acid peel to salicylic-mandelic acid peel (RR 1.06, 95% CI 0.88 to 1.26; 1 study, 40 participants; low-quality evidence), and we are uncertain if there is a difference in total minor adverse events due to very low-quality evidence (1 study, 44 participants). Neither group had withdrawals (2 studies, 84 participants; low-quality evidence).' No placebo-controlled or vehicle-controlled glycolic acid comparison is reported. Authors' conclusion for these comparisons: 'For other comparisons and outcomes the quality of evidence was low or very low. Risk of bias and imprecision limit our confidence in the evidence.' 26 of 49 studies at high risk of bias in at least one domain. SCOPE LIMITS: this is the strongest available synthesis for glycolic acid in acne and it finds the evidence base essentially empty — one small active-comparator trial.Funding / interest: All seven authors declare no conflicts of interest. Clinical referee Jerry Tan declares he is an advisor, consultant and/or investigator for Allergan, Almirall, Cipher, Galderma and Valeant.
  6. Garg S, Thami GP, Bhalla M, Kaur J, Kumar A. Comparative Efficacy of a 35% Glycolic Acid Peel Alone or in Combination With a 10% and 20% Trichloroacetic Acid Spot Peel for Melasma: A Randomized Control Trial. Dermatol Surg. 2019 Nov;45(11):1394-1400. PMID 31464703.Tier 2Supports: Randomised controlled trial, 30 patients with facial melasma randomly divided into 3 equal groups of 10. Group A: 35% glycolic acid full-face peel alone. Groups B and C: 35% glycolic acid full-face peel followed by a 10% or 20% TCA spot peel respectively. Four peels, one every 15 days. Outcome: percentage reduction in Melasma Area Severity Index (MASI). Results verbatim: 'All 3 groups had significant reduction of MASI, but there was no significant difference between the groups. Group A had minimum side effects.' Conclusion verbatim: 'Chemical peels with GA alone or in combination with TCA do result in a significant improvement in melasma, but the combination of the peels in the same sitting does not seem to have any additive or synergistic effect while they may increase the side effects.' SCOPE LIMITS: only 10 patients per arm; single centre (Government Medical College and Hospital, Chandigarh, India) so the population is predominantly Fitzpatrick III-V and the result should not be read across to phototype I-II without caution; there is NO untreated or vehicle control arm, so the absolute effect of 35% glycolic acid versus doing nothing is not established by this design; the abstract reports no numeric MASI values, no p-values and no confidence intervals.Funding / interest: No funding or conflict-of-interest statement retrievable from the PubMed record.
  7. Burns RL, Prevost-Blank PL, Lawry MA, Lawry TB, Faria DT, Fivenson DP. Glycolic acid peels for postinflammatory hyperpigmentation in black patients. A comparative study. Dermatol Surg. 1997 Mar;23(3):171-4; discussion 175. PMID 9145958.Tier 3Supports: Randomised pilot study. 19 patients with Fitzpatrick skin type IV, V or VI and postinflammatory hyperpigmentation randomised to control or peel group; 16 completed. Both groups applied 2% hydroquinone/10% glycolic acid gel twice daily plus 0.05% tretinoin cream at night. The peel group additionally received six serial glycolic acid peels at 68% maximum concentration. Assessed by photography, colorimetry and subjectively. Results verbatim: 'Both treatment groups demonstrated improvement, but the patients receiving the glycolic acid peels showed a trend toward more rapid and greater improvement. The peel group also experienced increased lightening of the normal skin.' Conclusion verbatim: 'This pilot study demonstrates that serial glycolic acid peels provide an additional benefit, with minimal adverse effects, for the treatment of postinflammatory hyperpigmentation in dark-complexioned individuals.' Background statement verbatim: 'there are no controlled studies examining their safety and efficacy in dark-complexioned individuals.' SCOPE LIMITS: 16 completers; the added benefit is described only as a 'trend', i.e. not statistically significant, and the authors' conclusion of 'additional benefit' is stronger than the reported result supports; peels were layered on top of hydroquinone plus tretinoin in both arms, so glycolic acid's independent contribution is confounded; 'increased lightening of the normal skin' in the peel group is an adverse cosmetic signal (uneven/perilesional hypopigmentation) in phototypes IV-VI, not a benefit.Funding / interest: No funding or conflict-of-interest statement retrievable from the PubMed record.
  8. Stiller MJ, Bartolone J, Stern R, Smith S, Kollias N, Gillies R, Drake LA. Topical 8% glycolic acid and 8% L-lactic acid creams for the treatment of photodamaged skin. A double-blind vehicle-controlled clinical trial. Arch Dermatol. 1996 Jun;132(6):631-6. PMID 8651713.Tier 2Supports: Single-centre, 22-week, double-blind, vehicle-controlled randomised clinical trial at Massachusetts General Hospital. 74 women aged 40-70 with moderately severe photodamaged facial skin. 8% glycolic acid, 8% L-lactic acid or vehicle cream applied twice daily to face and outer forearms. Primary result verbatim: 'The percentage of patients using either 8% glycolic acid or 8% L-lactic acid creams on the face achieving at least 1 grade of improvement (using a scale from 0 through 9) in overall severity of photodamage was significantly greater than with the vehicle cream (76% glycolic acid, 71% lactic acid, and 40% vehicle; P < .05).' Forearms at 22 weeks: glycolic acid superior to vehicle for overall severity of photodamage and sallowness (P<.05); L-lactic acid superior for overall severity, mottled hyperpigmentation, sallowness and roughness (P<.05). Withdrawals: 1 for skin irritation, 6 for personal reasons. Authors' own conclusion verbatim: 'Topical 8% glycolic acid and 8% L-lactic acid creams are modestly useful in ameliorating some of the signs of chronic cutaneous photodamage.' SCOPE LIMITS: women only, aged 40-70; a low-concentration leave-on cream, not a peel; the effect threshold is a single grade on a 0-9 scale; on forearms glycolic acid was superior on fewer endpoints than lactic acid; the authors themselves grade the benefit as 'modest'.Funding / interest: No funding or conflict-of-interest statement retrievable from the PubMed record.
  9. Ditre CM, Griffin TD, Murphy GF, Sueki H, Telegan B, Johnson WC, Yu RJ, Van Scott EJ. Effects of alpha-hydroxy acids on photoaged skin: a pilot clinical, histologic, and ultrastructural study. J Am Acad Dermatol. 1996 Feb;34(2 Pt 1):187-95. PMID 8642081.Tier 3Supports: Pilot study. Patients applied a lotion containing 25% glycolic, lactic OR citric acid to one forearm and a placebo lotion to the opposite forearm for an average of 6 months. Forearm skin thickness measured throughout; end-of-study biopsies from both forearms. Results verbatim: 'Treatment with AHAs caused an approximate 25% increase in skin thickness. The epidermis was thicker and papillary dermal changes included increased thickness, increased acid mucopolysaccharides, improved quality of elastic fibers, and increased density of collagen. No inflammation was evident.' Conclusion verbatim: 'Treatment with AHAs produced significant reversal of epidermal and dermal markers of photoaging.' SCOPE LIMITS: explicitly a pilot; sample size not stated in the abstract; forearm skin, not face; three different alpha-hydroxy acids are pooled as 'AHAs', so the glycolic-acid-specific effect cannot be separated; no blinding described; 25% is a leave-on concentration far above cosmetic norms and this is the source most often mis-cited as proof that consumer-strength glycolic acid builds collagen.Funding / interest: No funding or COI statement in the record. IMPORTANT COMMERCIAL INTEREST: co-authors Ruey J. Yu and Eugene J. Van Scott are the originators and patent-holders of alpha-hydroxy acid cosmetic technology and the founders of the NeoStrata skincare company, whose own corporate history states its founders 'unlocked an era-defining insight: Alpha Hydroxy Acids (AHAs) accelerate skin's natural turnover process' and 'patented the use of novel ingredients', with '100+ patents and publications' (https://www.neostrata.com/pages/our-story). They therefore had a direct commercial stake in the finding.
  10. Soleymani T, Lanoue J, Rahman Z. J Clin Aesthet Dermatol. 2018;11(8):21-28 — neutralisation statements appear in the glycolic, lactic, salicylic and pyruvic acid sections; there is no neutralisation statement anywhere in the mandelic acid section.Tier 4Supports: Full text read. Chemistry/penetration verbatim: glycolic acid 'is the smallest and simplest AHA in terms of chemical structure, and is also a highly hydrophilic molecule with the greatest bioavailability of all the AHAs.' Concentrations verbatim: 'For superficial chemoexfoliation, most concentrations range from 20 to 50%, with higher concentrations (70%) entering the medium-depth category.' Table 2 classifies glycolic 30-50% as superficial (depth: 'Intraepidermal and DE junction disruption possible') and 'Glycolic acid (70%, with or without pretreatment primer such as Jessner's solution)' as medium depth ('Full thickness epidermis into papillary dermis'). Over-the-counter agents described as '3-10% glycolic acid'. NEUTRALISATION verbatim: 'Glycolic acid is the prototypical non-self-neutralizing AHA (i.e., keratocoagulation will continue to occur as long as the caustic agent remains on the skin).' and 'Reaction completion of glycolic acid is achieved by introducing an alkaline neutralizing agent (e.g., sodium bicarbonate neutralization). Clinicians should note that neutralization of any acid with any base is an exothermic process; thus, patients should be warned that a transient increase in warmth, burning, or stinging will likely occur during neutralization.' CRUCIALLY, on which agents need neutralising: 'Salicylic acid, unlike glycolic acid and the other AHAs, is self-neutralized by the skin's own endogenous lipoproteins'; 'Unlike salicylic acid but much like other AHAs, pyruvic acid is not self-neutralizing and will continue to cause keratocoagulation for the duration of exposure to the skin until it is neutralized with an alkaline solution'; and for lactic and mandelic acid, 'As with glycolic acid, neutralization is necessary.' Frosting: superficial peel goal is 'little to no frosting (Level I at most)'; Level II+ 'indicates deeper destruction into the dermis and is not desirable'. Exfoliation over several days, re-epithelialisation complete within 7-10 days. SCOPE LIMITS: narrative review of the US market with no systematic methodology for its clinical assertions; concentration bands are expert practice convention, not trial-derived thresholds; contains no regulatory limits for any jurisdiction.Funding / interest: Stated verbatim: 'FUNDING: No funding was provided for this article. DISCLOSURES: The authors have no conflicts of interest relevant to the content of this article.'
  11. Khunger N; IADVL Task Force. Standard guidelines of care for chemical peels. Indian J Dermatol Venereol Leprol. 2008 Jan;74 Suppl:S5-12. PMID 18688104.Tier 4Supports: Consensus guideline of the Indian Association of Dermatologists, Venereologists and Leprologists Task Force. Neutralisation endpoint verbatim: 'For glycolic acid peels: The peel is neutralized after a predetermined duration of time (usually three minutes). However, if erythema or epidermolysis occurs, seen as grayish white appearance of the epidermis or as small blisters, the peel must be immediately neutralized with 10-15% sodium bicarbonate solution, regardless of the duration of application of the peel.' Contrast with other agents: TCA endpoint is frosting, neutralised with a neutralising agent or cold water; for salicylic acid 'the end point is the pseudofrost formed when the salicylic acid crystallizes', washed off with water after 3-5 minutes. Phototype caution verbatim: 'Superficial peels are considered safe in Indian patients. Medium depth peels should be performed with great caution, especially in dark skinned patients. Deep peels are not recommended for Indian skin.' Priming verbatim: 'It is essential to do prepeel priming of the patient's skin with sunscreens, hydroquinone and tretinoin for 2-4 weeks.' Contraindications listed include active bacterial/viral/fungal infection, keloid tendency, facial dermatitis, photosensitising medication and unrealistic expectations. Operator qualification verbatim: 'The physician performing chemical peeling should have completed postgraduate training in dermatology.' SCOPE LIMITS: an Indian guideline from 2008; the phototype guidance is framed around Indian skin (predominantly Fitzpatrick IV-V); the operator-qualification requirement reflects Indian professional structures and has NO legal force in the UK, where no equivalent statutory qualification requirement exists for chemical peels.Funding / interest: No funding statement in the record; produced by a professional association task force.
  12. US Food and Drug Administration. Guidance for Industry: Labeling for Topically Applied Cosmetic Products Containing Alpha Hydroxy Acids as Ingredients; Availability. Notice. Federal Register Vol. 70, No. 6, 10 January 2005, pp. 1721-1724. FR Doc 05-381.Tier 1Supports: Full text read. Quotes the 1998 Cosmetic Ingredient Review Expert Panel conclusion verbatim: 'Glycolic and Lactic Acid, their common salts and their simple esters, are safe for use in cosmetic products at concentrations <=10 percent, at final formulation pH >=3.5, when formulated to avoid increasing sun sensitivity or when directions for use include the daily use of sun protection. These ingredients are safe for use in salon products at concentrations <=30 percent, at final formulation pH >=3.0, in products designed for brief, discontinuous use followed by thorough rinsing from the skin, when applied by trained professionals, and when application is accompanied by directions for the daily use of sun protection.' This is the actual origin of the '10% / pH 3.5' consumer and 'professional' figures in circulation — and the professional figure is 30% at pH >= 3.0, NOT 70%. FDA's own position verbatim: 'The evidence reviewed so far by FDA suggests that topical application of a cosmetic product containing an AHA as an ingredient at any concentration may increase skin sensitivity to the sun and the possibility of sunburn.' Market survey verbatim: 'FDA analyzed approximately 100 cosmetic products containing AHAs as ingredients and found concentrations of AHAs ranging from 0.01 percent to 67 percent.' Adverse reports at very low levels: one product generating five adverse experience reports (skin irritation, burning) 'contained only 0.3 percent [alpha]-hydroxydecanoic acid and 0.4 percent [alpha]-hydroxyoctanoic acid, for a total of 0.7 percent AHAs'. On absorption: 'Percutaneous absorption studies suggest that topically applied AHAs in any cosmetic product may be absorbed by the skin to some extent'; 'While greater absorption was observed at pH 3, substantial absorption was observed at pH 7.' SCOPE LIMITS: United States only; FDA guidance documents are non-binding recommendations and carry NO legal weight in the UK; the recommended labelling is a sunburn-alert statement, not a concentration restriction.Funding / interest: The concentration/pH conclusion quoted is the CIR Expert Panel's, not FDA's own. FDA expressly DECLINED to adopt it as a basis for exempting compliant products from sunburn-alert labelling ('FDA has not modified the guidance in response to this comment').
  13. National Center for Biotechnology Information. PubChem Compound Summary for CID 757, Glycolic acid. Retrieved via PubChem PUG-REST API, 1 August 2026.Tier 4Supports: Queried directly via the PUG-REST computed-properties endpoint. Returns: CID 757; MolecularFormula C2H4O3; MolecularWeight 76.05 (g/mol); IUPACName '2-hydroxyacetic acid'; XLogP -1.1; SMILES C(C(=O)O)O. Synonym list confirms CAS 79-14-1 and the alternative names 'hydroxyacetic acid', 'glycollic acid', '2-hydroxyethanoic acid', 'alpha-hydroxyacetic acid'. The negative XLogP of -1.1 is the quantitative basis for describing glycolic acid as strongly hydrophilic. SCOPE LIMITS: curated/computed physicochemical values only; carries no biological, clinical or penetration claim. The query did NOT return a pKa value, so no pKa is asserted here.
  14. Sitohang IBS, et al. Water as a substitute for sodium bicarbonate solution in neutralising a 35% glycolic acid peel: a randomised, double-blind, split-face, controlled trial. 2021.Tier 2Supports: REFUTES THE ABSOLUTE. A randomised, double-blind, split-face controlled trial in 126 healthy patients aged 18-60 with Fitzpatrick phototypes IV-V. One facial half was neutralised with WATER after a 35% glycolic peel; the other with sodium bicarbonate solution. Water was found to be an effective substitute; there was a significant difference in pain score immediately after neutralisation FAVOURING WATER, with no meaningful difference in erythema or pruritus. SCOPE: one 35% formulation, phototypes IV-V. It does not license water for every glycolic product, and it does not mean exposure need not be stopped.
  15. Sharad J. Glycolic acid peel therapy - a current review. Clinical, Cosmetic and Investigational Dermatology. 2013;6:281-288.Tier 4Supports: DEPTH DEPENDS ON TIME, NOT ONLY CONCENTRATION. Citing Fabbrocini's 2009 classification: SUPERFICIAL is '50%-70% GA, applied for 2-5 minutes'; MEDIUM DEPTH is '70% GA, applied for 3-15 minutes'. The same 70% is superficial at short contact and medium-depth at long contact. Verbatim on what governs intensity: 'The intensity of peel is determined by the concentration of the acid, the vehicle used to carry it, the amount of acid applied, and the technique used.' ON FROSTING, verbatim: 'If frosting is observed in any particular area before the set time or end-point, it is important to neutralize the peel immediately.' Frosting is a STOP SIGNAL in glycolic-specific guidance, not a target.
  16. National Toxicology Program. NTP Technical Report on the photocarcinogenesis study of glycolic acid and salicylic acid in SKH-1 mice. NTP TR 524.Animal studyTier 3Supports: THE DIRECT ANIMAL REPORT, rather than a regulatory summary of it. SKH-1 hairless mice, 4% or 10% glycolic acid cream at pH 3.5 against control cream, with simulated solar light from a filtered xenon arc at 0.0, 0.3 or 0.6 minimal erythema dose, five days a week for 40 weeks. CONCLUSION: 'glycolic acid did not alter the photocarcinogenesis of SSL, and salicylic acid was photoprotective.' ONE EXCEPTION: a glycolic-acid dose trend in squamous-cell-carcinoma incidence among MALE mice at 0.3 MED. Animal data cannot establish or exclude a human cancer risk in either direction.

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