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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 glycolic acid is, and the preparations available#

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 reaches the treatment room in two quite different preparations. Cosmetic and over-the-counter products sit at roughly 3–10%, although a US market survey of about 100 AHA products found total AHA concentrations spanning 0.01% to 67%.[10, 12]Professional peel solutions run roughly 20–70%.[10, 15]

The two are measured on different evidence bases and behave differently on skin, which makes the distinction the one that prevents most of the errors in this area. A trial of an 8% twice-daily leave-on cream is not evidence for a 35% peel, and the strength printed on a professional bottle does not by itself describe the depth of the treatment.

Use of glycolic acid in aesthetic practice#

Both the retail product and the professional peel solution are regulated as cosmetics, and what governs them is a general obligation rather than an ingredient-specific number. A Responsible Person must hold a Cosmetic Product Safety Report, the finished product must meet the cosmetic definition, and it must be safe in normal or reasonably foreseeable use. That is a real constraint. It is simply not a figure that can be quoted as a legal maximum.

Concentration and pH limits in Great Britain and the EU

Glycolic acid appears in neither Annex II nor Annex III of the EU Cosmetics Regulation as consolidated to 1 May 2026, so no maximum concentration and no minimum pH are set for it, for consumer or professional products. Northern Ireland follows that regime under the Windsor Framework.[2]

The GB assimilated Annex III is the same, so no statutory concentration or pH limit applies in Great Britain either.[3] Every apparent hit in both texts is thioglycolic acid, an unrelated substance used in perming and depilatories.

Absence from a restricted-substance annex is a narrower finding than it first appears. It is not permission for any strength, pH, claim or use, and a product’s classification can still change with the claims made for it, its purpose and its mode of action.

Qualification and licensing across the four nations

Statutory qualification, premises and local-authority licensing for chemical peels are set nation by nation rather than UK-wide. Scotland has a 2026 framework Act in place with the detail still to come in regulations, and local-authority licensing varies elsewhere. No mandatory qualification for a glycolic acid peel is named in the sources reviewed here.[no source found]

Contraindications and cautions#

Fitzpatrick IV–VI skin

The most-cited guidance for darker skin is a 2008 Indian expert consensus written 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.[11]

Its reach is narrower than its citation count suggests. The document reports no study cohort and no Fitzpatrick denominator at all, so its population cannot be inferred from geography: it is a consensus for another country’s physicians rather than a UK guideline or a phototype V–VI trial.

The one study conducted in this group carries a caution of its own. The peel arm developed lightening of the surrounding normal skin alongside the intended effect.[7]The risk to consent for in phototypes IV–VI is therefore uneven lightening of untreated skin as well as post-inflammatory hyperpigmentation.

Clinical uses and the evidence behind them#

Photoageing

This is the strongest indication, and it rests on 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 features of that trial travel with the result. The lactic acid arm did nearly as well as the glycolic one, so the finding is not evidence of a glycolic-specific advantage. And the product was an 8% twice-daily leave-on cream rather than a peel, which means nothing in it transfers to a 35% or 70% professional treatment.

Melasma

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]

Both halves of that carry a ceiling. Within-group change with no untreated or vehicle arm cannot establish 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.[6]

Post-inflammatory hyperpigmentation in darker skin

The thinnest evidence for glycolic acid sits on the question where the stakes are highest. In post-inflammatory hyperpigmentation in Fitzpatrick IV–VI skin, adding six serial glycolic peels at a 68% maximum concentration to a hydroquinone-plus-tretinoin regimen produced what the authors describe only as a trend toward more rapid and greater improvement in 16 completers, with no supporting statistics reported.[7]

Both arms received hydroquinone and tretinoin, so the peels’ independent contribution is confounded, and a described trend is not a demonstrated benefit.

Acne vulgaris

Cochrane included fiveglycolic-acid peel studies using 20–70%, so the literature is larger than it is often said to be. 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, and no placebo- or vehicle-controlled glycolic acid comparison appears anywhere in the review’s results.[5]

That is an absence of evidence rather than evidence of no effect. A glycolic peel cannot honestly be described to a client as evidence-based acne care, and photoageing remains the indication it can be offered for on the evidence above. Where a superficial peel is being chosen for acne specifically, the comparative evidence between peel agents is set out under salicylic acid.

Selecting a preparation and setting the exposure#

Percentage alone does not determine depth. A widely used classification puts 50–70% applied for two to five minutes in the superficial category and 70% applied for three to fifteen minutesin the medium-depth one — the same number, on either side of the line. Intensity is determined by the concentration, the vehicle, the amount applied and the technique together.[10, 15]

The working consequence is that a treatment is described by its formulation, its protocol and its achieved endpoint rather than by its percentage. A 70% preparation is a strength that can reach medium depth rather than one that always does, and the consent conversation follows the protocol.

Ending the exposure

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]

The stopping method is formulation- and protocol-specific rather than one alkaline recipe. A randomised, double-blind, split-face trial in 126 adults with phototypes IV–V found water an effective substitute for sodium bicarbonate after a 35% glycolic peel, with significantly lesspain immediately after neutralisation and no meaningful difference in erythema or pruritus. The manufacturer’s validated instructions are the operative ones for a given product.

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 consensus written for physician-led dermatology practice. Epidermolysis looks like greyish-white epidermis or small blisters. The stop-on-epidermolysis half of that guidance is sound and worth keeping; the recipe and the three minutes belong to that document rather than to UK law, a dose-ranging trial or any given product’s instructions for use.[11]

Neutralisation across the acid families

The International Peeling Society expert review states 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]

The International Peeling Society, set out under chemical peels, holds that only glycolic and pyruvic acid require neutralisation. Two expert sources therefore disagree about lactic acid. The stopping protocol follows the instructions for the exact product in use; where those are silent, the conservative position is to assume an alpha-hydroxy acid needs neutralising unless its manufacturer says otherwise, and to treat salicylic acid as the reliable exception. A salicylic acid habit does not carry across to an AHA.

Endpoints and frosting

For a superficial glycolic peel the endpoint is the product-specific time and clinical endpoint rather than 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]

Frosting is a depth signal on a TCA peel and an overshoot alarmon a superficial glycolic peel — the same observation with opposite meanings, depending on the agent in use. Level-I frosting as a target, and seven to ten days as a healing time, come from a generic superficial-peel scale rather than from anything glycolic-specific.

Adverse effects and their management#

Sensation at the moment of neutralisation

Acid-base neutralisation is exothermic, so there is a transient surge of warmth, burning or stinging at exactly the moment the client expects relief.[10] It is expected rather than a sign that the peel has gone wrong, and an unwarned client reads it as the second.

Ultraviolet sensitivity

This is 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]

The effect resolves. No difference in sunburn cells or minimal erythema dose remained one weekafter stopping. That is a washout for that model — a 10% leave-on cream on back skin — and photoprotection after a professional peel has never been studied.[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] The committee itself 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, so the position rests on precaution as much as on measurement.[1]

Photocarcinogenesis

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, with one exception: a dose trend in squamous-cell-carcinoma incidence among male mice at 0.3 minimal erythema dose. It is animal work with no sunscreen-only control, so it neither establishes nor rules out a human risk, and it is not reassurance that can be offered to a client.[1, 16]

Reducing, stopping and referring

Exposure is stopped at once, whatever the timer says, where visible whitening, erythema or grey-white epidermolysis appears before the planned endpoint. Between sessions the adjustable variables are the strength, the contact time and the interval, none of which has a dose-ranging trial behind it. Referral is indicated where an over-peel does not settle, or where pigment change follows in phototypes IV–VI, rather than a further session at a reduced strength.

Referral and scope boundaries#

Neither the retail product nor the professional peel solution is a prescription-only medicine, which makes referral here a matter of the plan and of the response to treatment rather than of the agent. Referral is indicated in three situations:

  • A plan that calls for prescription priming.The regimen the most-cited darker-skin guidance calls essential — two to four weeks of sunscreen, hydroquinone and tretinoin before the peel course — is prescriber-led in UK terms. 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.[11] The consequence is a sequencing one: the prescriber is needed before the peel course begins, not after it.
  • An over-peel that does not settle. Erythema, greyish-white epidermolysis or blistering persisting after the exposure has been stopped is beyond the course of a superficial peel, and no glycolic-specific healing time exists against which to wait it out.
  • Pigment change in phototypes IV–VI. Post-inflammatory hyperpigmentation, or lightening of the surrounding untreated skin, in a group for which no complication rate broken down by phototype has been published.

Photoprotection is the part of that priming regimen that needs no prescription, and it continues alongside a prescriber rather than waiting on one.

Mechanism of action#

The expert review most often cited describes glycolic acid, in a single breath, as the smallest and simplest AHA, a highly hydrophilic molecule, 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]

On the skin, keratocoagulation continues for as long as the acid remains in contact, which is the property that separates glycolic acid from the self-neutralising agents and the reason contact time features in every classification of peel depth.[10, 11, 14]

Commonly misstated claims#

Four statements circulate widely in the trade. Each is set out below with what the primary literature was found to support.

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

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 rather than 3.5, and it was never written into the Annexes.[1, 2]

Supported statement: a non-binding scientific committee recommendation exists at 4% and pH 3.8; no legal maximum concentration or minimum pH does.

“Professional glycolic acid is capped at 70%.”

The only quantified professional figure located comes from the US Cosmetic Ingredient Review panel, and it is 30% at pH 3.0 rather than 70%. 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 are non-binding even in the United States.[12]

Supported statement:70% is the upper end of professional convention, and the only published professional number is a US industry panel’s 30% at pH 3.0, which carries no UK weight.

“Glycolic acid penetrates best because it is the smallest AHA.”

The bioavailability ranking is an expert-review assertion, presented without a head-to-head human penetration result behind it.[10] No head-to-head human study comparing the skin penetration of the alpha-hydroxy acids against one another was located, and a source asserting a ranking cannot itself establish that no such study exists.[no source found]

Supported statement: an expert review ranks glycolic acid highest for bioavailability among the AHAs, and the molecular-weight explanation for it is chemically plausible but has not been demonstrated comparatively in people.

“The classic study shows that an 8% glycolic serum builds collagen.”

The histology behind that statement is not glycolic-specific. A 17-person split-forearm pilot allocated five participants to glycolic acid, five to lactic and seven to citric, then pooled the results across all three and biopsied eight people. It reported roughly a 25% increase in skin thickness with increased dermal collagen density, using a 25% leave-on lotion on forearm skin. Two of its authors were the patent-holders and company founders behind AHA skincare.[9]

Supported statement: a pooled AHA pilot reported dermal changes at 25% on forearm skin; it cannot isolate a glycolic-specific effect and does not speak to consumer-strength products.

Areas of remaining uncertainty#

  • 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. It is worth re-checking before it is relied on commercially.
  • Statutory qualification, premises and local-authority licensing requirements for chemical peels differ across the four UK nations, and no source reviewed here names a mandatory qualification. The position in a given nation and licensing area needs confirming locally.
  • No trial establishes an optimal exposure time, neutralisation timing or session interval. The three-minute convention is consensus rather than a dose-ranging result, which leaves the individual product’s instructions as the operative guidance.
  • No RCT of glycolic acid has been conducted specifically in Fitzpatrick V–VI, and no complication rates broken down by phototype have been published, so phototype-specific risk cannot be quantified.
  • No placebo- or vehicle-controlled trial of a glycolic peelexists for any indication — every peel trial located used an active comparator — so the absolute effect of a peel, as distinct from its effect relative to another acid, remains unmeasured.
  • 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.

Frequently asked questions#

What concentration is legally permitted?

The Cosmetics Regulation sets no ingredient-specific limit for glycolic acid at all.[3]What constrains the choice in practice is insurance, qualification, the supplier’s instructions and the product’s safety assessment, none of which is the same thing as a statutory cap.

How long is a glycolic peel left on?

Convention is about three minutes, and the operative answer is the product’s stated time or the clinical endpoint, whichever comes first. Erythema or epidermolysis means the peel is neutralised immediately, regardless of the timer.[11]

How long does sun sensitivity last after a peel?

It has not been measured after a peel. What was measured is a 10% leave-on cream on the backs of 29 participants: the excess sunburn-cell induction and the minimal-erythema-dose difference were no longer detected a week after stopping. Treating that as a post-peel window is a precaution borrowed from a different model rather than a result.[4]

Is glycolic acid established as safe on Fitzpatrick V–VI?

Not established. 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 and gave no V–VI subgroup.[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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