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Salicylic acid

Also known as: salicylic acid, BHA, beta-hydroxy acid, SA peel

Salicylic acid is lipid-soluble 2-hydroxybenzoic acid, marketed as a BHA but structurally a phenolic aromatic acid. Used at 0.5–2% in cosmetics and 20–30% as a professional peel, it is desmolytic rather than keratolytic and is removed at a timed endpoint rather than neutralised.

Evidence status

Moderate

For acne, salicylic acid peeling is supported by a systematic review of 12 RCTs (387 participants, Cochrane methodology, published in BMJ Open) plus small split-face RCTs — but that review rates included-trial quality as 'very low to moderate', could not meta-analyse, and finds SA broadly EQUIVALENT to other superficial peels rather than superior. The vehicle pharmacology that practitioners are taught (pseudofrost, self-limiting crystallisation, SA-HA hot spots, PEG slowing delivery) rests almost entirely on one expert narrative review by the International Peeling Society; the underlying primary paper for the SA-PEG comparison is paywalled and its abstract does not describe the split-face design the review attributes to it. Safety in Fitzpatrick V-VI rests on a single 25-patient uncontrolled pilot confounded by hydroquinone pre-treatment. Systemic-safety evidence is genuinely reassuring for facial peeling (n=9 crossover PK study, 50:1 AUC safety margin against 650 mg aspirin) but that study was run by L'Oreal employees. The salicylism literature is real but is entirely about large-body-surface keratolytic use in psoriasis and ichthyosis, not peels.

What salicylic acid is, and the forms in use#

Salicylic acid is lipid-soluble 2-hydroxybenzoic acid, used at 0.5–2% in cosmetic products and at 20–30% as a professional peel. Yu and Van Scott classify it as a phenolic aromatic acid; Kligman described it as a beta-hydroxy acid, and that is the name under which it is marketed throughout skincare.[1]

Lipid solubility is the property that matters clinically. Salicylic acid is miscible with epidermal and sebaceous-gland lipids, which is why it reaches the sebum-filled follicle and why its clearest use is comedonal acne rather than general exfoliation.

The hydroalcoholic and polyethylene glycol vehicles

Two professional preparations are in use, and they are distinguished by vehicle rather than by concentration. More than for any other peel agent, what salicylic acid is dissolved in determines what it does.

In an ethanol (hydroalcoholic)vehicle at 20–30%, the acid crystallises as the ethanol evaporates, and within about 30 seconds to a minute a white precipitate appears on the skin.[1, 9] A polyethylene glycol vehicle behaves differently: it slows delivery while increasing follicular penetration. They are different treatments carrying the same name and the same percentage.

Use of salicylic acid in aesthetic practice#

Salicylic acid is sold both as a consumer cosmetic and as a professional peel solution, and the two sit in different regulatory positions. Cosmetic products within the Great Britain caps are yours to recommend and retail. A 20–30% peel is a professional procedure, and the regime that governs a solution at that strength is the unresolved question below.

Cosmetic concentration limits in Great Britain

In Great Britain, salicylic acid is capped in cosmetic products at 3% in rinse-off hair products and 2% in other products (excluding body lotion, eye shadow, mascara, eyeliner, lipstick and roll-on deodorant) under Annex III entry 98, and at 0.5% as a preservative under Annex V entry 3, with a mandatory warning against use in products for children under three.[10]

Those caps govern products placed on the GB market as cosmetics. How OPSS or the MHRA classifies a professional-use 20–30% peel solution — whether it sits inside that regime and its caps, or outside them — could not be established from the sources reviewed here, so neither the 2% cap nor an exemption from it may be assumed. Northern Ireland follows the EU cosmetics regime under the Windsor Framework and is checked separately.[10]

Licensing of chemical peeling in the United Kingdom

No UK-wide statutory scheme names salicylic acid peels, sets a maximum professional concentration or treated body-surface-area limit, or specifies a practitioner qualification for chemical peeling. England’s 2023 licensing consultation proposed a tiered model but no scheme is in force; Wales licenses only its four special procedures, which do not include peels; and whether peels fall within Scotland’s 2026 Act will depend on the regulations made under it. Local-authority special-treatment licensing may apply in some areas.[no source found]

The reproductive toxicity classification under CLP

Salicylic acid was classified as a Category 2 reproductive toxicant (H361d)under CLP in 2018. That is a substance-level hazard classification, and it is what triggered the industry derogation and the UK scientific advisory group’s January 2022 opinion — which concluded there would be no appreciable increase in health risk at the cosmetic concentrations under consideration.[10]

A hazard classification is not a use-level risk assessment, and it does not by itself establish that a professional peel is unsafe in pregnancy. It is nonetheless the operative UK reference point for the substance.

Contraindications and cautions#

Pregnancy and planned pregnancy

No study of topical salicylic acid in pregnancy has been conducted, and none of salicylic acid peels specifically.[11, 9] The reassurance usually offered is an inference from low systemic absorption: because a relatively small proportion is absorbed through the skin, topical salicylic acid is judged unlikely to pose a risk, and large studies of low-dose aspirin in pregnancy found no increase in major malformations, preterm birth or low birth weight.

Against that, the peel literature recommends avoiding salicylic acid peels in pregnancy on grounds of structural similarity to aspirin and a US FDA pregnancy category C label — a labelling system that no longer exists.

Deferring an elective peel until after pregnancy is a reasonable and defensible default. It is caution in the absence of data rather than a documented contraindication, and a retired US labelling category is not a finding that supports it.

Compromised skin barrier

Excoriated, eczematous or freshly-treated skin is a different absorption problem from healthy skin. In hairless mice, 30% salicylic acid in a PEG vehicle produced plasma radioactivity of about 1,665 ng eq/mL one hour after application to intact skin, against about 21,438 ng eq/mL through damagedskin — roughly a thirteen-fold difference. In the carcasses remaining after the treated skin was removed, 0.09% of the applied dose was found after intact-skin application against 11.38% after damaged-skin application.[4]i

That is mouse data, and hairless mouse skin is more permeable than human skin, so it reads as direction and magnitude rather than as a human absorption figure. Barrier integrity is also one determinant among several: concentration, vehicle, pH, quantity, treated area and contact time all bear on exposure.

Clinical uses and the evidence behind them#

Acne vulgaris

A systematic review covering 12 randomised trials and 387 participants found salicylic acid statistically indistinguishable from trichloroacetic acid (RR 0.89, 95% CI 0.73–1.10), from glycolic acid (RR 1.00, 0.85–1.18) and from pyruvic acid (RR 1.11, 0.73–1.69) for mild-to-moderate acne.

Each of those head-to-heads rests on a single small trial, no meta-analysis was possible, and the reviewers’ own summary is that commonly used peels appear similarly effective and well tolerated on evidence of very low to moderate quality.[5] No difference detected on that base is a weaker finding than equivalence, and it sets the ceiling on what the acid may be offered for.

Comedones

This is where a specific edge appears. In a 40-patient randomised trial, six fortnightly 30% salicylic acid peels reduced comedones by 53.4% against 26.3%for Jessner’s solution (p=0.001), with equivalent results for papules and pustules and a greater fall in Michaelson Acne Score (60.4% vs 34.1%, p=0.002). The reviewers hedge it as “may be superior”.[5]

A separate 22-participant trial found weekly 10% salicylic acid — well below peel strength — no different from phototherapy for comedones or papules, but less effective for pustules. That concerns a low-strength topical protocol and is not a finding about 20–30% professional peels.

Comparison with glycolic acid

In a split-face, double-blind randomised trial of 20 patients receiving six fortnightly treatments, 30% salicylic acid and 30% glycolic acid were similarly effective and both worked by the second treatment — but salicylic acid had sustained effectiveness at two months.[6]

Twenty patients is small. Durability rather than peak effect is the reasonable basis for preferring salicylic acid in an acne-prone client, and it is a single trial’s worth of basis.

Darker skin phototypes

Salicylic acid is described in the peel literature as safe and well tolerated in all Fitzpatrick skin types I–VI, and in types V–VI that description rests on preliminary data.[1, 2] It is therefore a plausible option in higher phototypes rather than a demonstrated comparative preference over another superficial peel.

Selecting a preparation and timing the peel#

Selection turns on the vehicle rather than on the percentage, and the comparative evidence between the two vehicles does not settle it. The split-face comparison usually cited for the polyethylene glycol preparation is not verifiable at source, so the choice rests on the preparation in use rather than on demonstrated superiority.[1, 3]

Contact time

The hydroalcoholic peel is left on for its intended contact time — commonly three to five minutes — and rinsed off with water.[1, 9]

The clock is the control, not the crystal. The peel is widely taught as self-limiting on the grounds that the crystals cannot penetrate, but that is an expert assertion in a narrative review with no penetration study attached to it, and it does not substitute for timing the peel.[1, 9]

The pseudofrost

Salicylic acid frost is precipitated salicylic acid. Trichloroacetic acid frost is coagulated skin protein. They are not the same physical event, and an SA pseudofrost must never be read as a TCA depth reading.[9, 1]

It carries information all the same. The peel literature treats visible frost as the signal that the peel is complete, and notes that once frosting has occurred there will be some crusting and peeling — which is why, when treating melasma or other pigment dyschromias, minimal or no frosting is the preferred endpoint.[9, 1]

The workable position is to time the peel and read the frost as a completion and vigour cue. Heavy frosting on a pigmentary indication records a peel more vigorous than that indication wanted.

Neutralisation

Salicylic acid peels do not require neutralisation. The frost appears when the peel is complete and the residue is simply rinsed off with water — stated explicitly in the literature as an advantage over alpha-hydroxy acid peels.[1, 9] The International Peeling Society extends the same point to tretinoin and to Jessner and modified Jessner solution, and holds that among common superficial peels only glycolic and pyruvic acid require neutralisation.

The wider list is contested. It appears in a paywalled narrative review that could not be verified at source, and the International Peeling Society expert review has lactic and mandelic acid requiring neutralising too. The stopping protocol therefore follows the instructions for the exact product in use, and a salicylic acid habit does not carry across to an alpha-hydroxy acid.

Adverse effects and systemic exposure#

Expected local response

Crusting and peeling follow visible frosting.[9, 1]

In the split-face comparison against 30% glycolic acid, more adverse events were reported on the glycolic side after the initial treatment.[6] That is a twenty-patient observation about the first treatment in a course, not a general tolerability ranking.

Systemic absorption and salicylism

Salicylism — tinnitus, nausea, confusion — is real, and the literature describing it concerns large-body-surface keratolytic use in psoriasis and ichthyosis rather than facial peeling.

In nine healthy subjects, a 30% salicylic acid facial peel left on for five minutes produced a mean peak plasma salicylic acid of 0.81 µg/mL, against 56.4 µg/mLafter a single 650 mg oral aspirin — a safety margin of about 50:1 on area under the curve. There was a depot effect, with absorption continuing after the peel was removed.[8]

Whether any facial or peel-associated case of salicylism has ever been reported could not be established, because the case tables behind that claim were not inspectable.[8]

Reducing, stopping and referring

Contact time is the variable that is reduced. On a pigmentary indication minimal or no frosting is the preferred endpoint, so frosting heavier than that records a peel to be run more lightly next time rather than a result achieved.[9, 1]

Where the barrier is open the peel is not run at all until it has healed. Excoriated, eczematous or freshly-treated skin is a different absorption problem rather than a more sensitive version of the same one.[4]i

A reaction that has not settled, or a presentation that turns out to need a diagnosis rather than a peel, belongs with a clinician.

Referral and scope boundaries#

Salicylic acid peeling is a cosmetic procedure, and the decisions that sit outside it are the ones worth naming. Referral or deferral is indicated in three situations:

  • Pregnancy or planned pregnancy. No study of topical salicylic acid in pregnancy has been conducted, so an elective peel is deferred as a precaution rather than because a contraindication has been documented.[11, 9]
  • Excoriated, eczematous or freshly-treated skin. A broken barrier changes the absorption question, and the skin condition itself may need treating before any peel is considered.[4]i
  • Pigmentation of unclear cause.The evidence in Fitzpatrick V–VI is preliminary, and an undiagnosed dark mark is a diagnostic question first.[1, 2]

None of the three is a permanent bar. Each is a question that belongs elsewhere first, and the peel is deferred until it has been answered.

Mechanism of action#

Salicylic acid is desmolytic. It extracts desmosomal proteins, including desmogleins, so cell-to-cell cohesion is lost and the stratum corneum sheds, rather than lysing intercellular keratin filaments. The exact mechanisms are still not fully characterised.[9]

The distinction explains the behaviour. Salicylic acid does not dissolve tissue; it unglues it. That is why the shedding is orderly, why the peel is comparatively forgiving, and why it can be used at strengths that would be alarming in an acid that worked by coagulation.

Concentration and the anti-inflammatory effect

The anti-inflammatory action is described as most pronounced at 0.5–5% w/w— an order of magnitude below the 20–30% used in a professional peel.[9]

On that account the calming effect of a 2% leave-on product and the exfoliation of a 30% peel are different mechanisms operating in different concentration ranges, rather than one mechanism turned up.

Commonly misstated claims#

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

“Salicylic acid is a beta-hydroxy acid.”

In a true beta-hydroxy acid the hydroxyl group sits on an aliphatic carbon chain. In salicylic acid the carboxyl and hydroxyl groups attach directly to an aromatic benzene ring, which is why Yu and Van Scott classify it as a phenolic aromatic acid. Kligman described it as a BHA and the trade name stuck.[1]

Supported statement:salicylic acid is lipid-soluble — the property the BHA label is usually invoked to convey — and is structurally a phenolic aromatic acid.

“A split-face study showed 30% salicylic acid in PEG beating the hydroalcoholic vehicle.”

The claim comes from a narrative review. The primary paper it cites reports an uncontrolledevaluation in 44 volunteers and 436 acne patients plus mouse histology — with no split-face design, no comparator arm and no effect size in its published abstract, and the full text paywalled.[1, 3]

Supported statement: the polyethylene glycol vehicle slows delivery and increases follicular penetration; no controlled comparison establishes that it treats acne better than the hydroalcoholic one.

“Salicylic acid is the most effective superficial peel for acne.”

The reviewers decline both superiority and equality among the common superficial peels, on evidence they grade very low to moderate.[5]The one place a specific edge appears is comedones, and it is hedged as “may be superior”.[5]

Supported statement:no superficial peel has been shown to outperform another for mild-to-moderate acne, and salicylic acid’s comedone advantage rests on a single trial.

“Salicylic acid peels are safe in all Fitzpatrick skin types.”

The statement appears in the peel review literature, and its sole citation for types V–VI is one uncontrolled 25-patient pilot.[1, 2] Every patient in it was primed with hydroquinone 4% for two weeks. Moderate to significant improvement was recorded in 88% and minimal to mild side effects in 16% — whole-sample figures across a mixed sample of nine with acne, five with post-inflammatory hyperpigmentation, six with melasma and five with rough, oily skin, with no comparator arm.[2]

Supported statement:a small uncontrolled series in Fitzpatrick V–VI went well on hydroquinone-primed skin; nothing was compared, so the pilot cannot rank salicylic acid against another superficial peel in darker skin.

Areas of remaining uncertainty#

  • Whether the peel is genuinely self-limiting. The claim is an expert assertion with no penetration study attached, which leaves the clock rather than the crystal as the control on contact time.
  • Whether the polyethylene glycol vehicle outperforms the hydroalcoholic one. The study cited for it does not describe the design attributed to it, so neither vehicle can be presented to a client as the better one.
  • Safety and pigmentary outcomes in Fitzpatrick V–VI, where the evidence is a single uncontrolled pilot confounded by hydroquinone pre-treatment — which makes it an option offered on preliminary data rather than a comparative preference.
  • Anything about pregnancy. No study of topical salicylic acid in pregnancy exists, which makes deferral a precaution rather than the observance of a contraindication.
  • How a professional-use 20–30% peel solution is classified in Great Britain, so that neither the 2% cosmetic cap nor an exemption from it may be assumed.
  • Optimal contact time, session interval and course length, all of which are convention rather than findings, so a stated course length carries no evidential weight.

Frequently asked questions#

Does a salicylic acid peel require neutralising?

No alkaline neutraliser is used. The residue is rinsed off with water once the contact time is up, and the same reference text puts tretinoin, Jessner and modified Jessner solution in the same group.[1, 9]

How long is a salicylic acid peel left on?

Commonly three to five minutes. The peel is timed rather than run to a visual cue, and on a pigmentary indication the target is minimal frosting.[1, 9]

Is salicylic acid suitable for a client with Fitzpatrick VI skin?

It is a plausible option rather than a demonstrated first choice. The evidence in Fitzpatrick V–VI is a single uncontrolled series on hydroquinone-primed skin, set out under commonly misstated claims above.[2]

Can a peel be applied where a client has facial eczema?

Not to the affected skin. In hairless mice, one-hour plasma radioactivity after a 30% PEG formulation was about thirteen-fold higher through experimentally damaged skin — an animal model showing direction and magnitude, not a human multiplier — and eczematous skin is also more reactive.[4]i

What strength may be bought and used in Great Britain?

The caps — 2% in most products, 3% in rinse-off hair products, 0.5% as a preservative — apply to products regulated as cosmeticsin Great Britain. How a professional-use 20–30% peel solution is classified could not be established, so neither the 2% cap nor an exemption from it may be assumed, and Northern Ireland follows the EU cosmetics rules under the Windsor Framework.[10] Separately, no UK-wide statutory scheme sets a maximum concentration or a mandated qualification for chemical peeling.[no source found]

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. Lee KC, Wambier CG, Soon SL, Sterling JB, Landau M, Rullan P, Brody HJ; International Peeling Society. Basic chemical peeling: Superficial and medium-depth peels. J Am Acad Dermatol. 2019 Aug;81(2):313-324. doi:10.1016/j.jaad.2018.10.079. PMID 30550830.Tier 4Supports: CME narrative review authored by the International Peeling Society (group byline); not a systematic review and no formal grading of evidence. Full text read via the IPS-hosted PDF. Verbatim: 'SA is a beta-hydroxy acid and a phenolic compound with antiinflammatory, antimicrobial, and depigmenting properties. It is safe in skin of all Fitzpatrick phototypes. Because of SA's lipophilic and comedolytic effects, it is particularly effective for comedonal acne. SA 20% to 30% in ethanol (hydroalcoholic vehicle [HA]) crystallizes upon ethanol evaporation, yielding a pseudofrost that can be removed from the skin with a facewash or wet cloth, if desired. The crystals cannot penetrate the skin; the peel is thus self-limiting. Some patients develop SA-HA hot spots, or areas of overpenetration that may result in postinflammatory hyperpigmentation (PIH). In response, a polyethylene glycol (PEG) vehicle was developed that slows delivery while simultaneously increasing follicular penetration. Typically, the SA-PEG peel is left on the skin for >=5 minutes to allow for follicular penetration, and then rinsed off with water as the PEG vehicle is occlusive. A split-face study for acne showed superiority of 30% SA-PEG to 30% SA-HA. SA-HA peels yield mild desquamation after 2 days. In contrast, SA-PEG usually does not cause desquamation. Salicylic acid peels may cause urticaria and angioedema, with known cross-sensitivity to acetylsalicylic acid.' Also verbatim, on neutralisation: 'Tretinoin, salicylic acid, Jessner solution, and modified Jessner solution do not require neutralization' and 'Glycolic and pyruvic acid require neutralization'; and 'Of these, only GA and PA peels require neutralization, either by sodium bicarbonate or by removal with water.' On acid class: 'Alpha-hydroxy acids, such as glycolic acid, are water soluble. Beta-hydroxy acids, such as salicylic acid, are lipid soluble.' On darker skin: 'In darker skin types, superficial peels are safe and effective in reducing papule, pustule, and comedone count.' SCOPE LIMITS: the 'safe in skin of all Fitzpatrick phototypes' statement is referenced to a single 25-patient uncontrolled pilot (Grimes 1999); the anti-inflammatory reference is a rat carrageenin pleurisy study (Chiabrando 1989), the antimicrobial reference is a Pseudomonas aeruginosa membrane-proteome in-vitro study (Bandara 2016), and the depigmenting reference is a tyrosinase-inhibitor review (Chang 2009) — none is a clinical trial of peel-strength SA on human skin. No funding statement or COI disclosure appears in the article PDF text.
  2. Grimes PE. The safety and efficacy of salicylic acid chemical peels in darker racial-ethnic groups. Dermatol Surg. 1999 Jan;25(1):18-22. doi:10.1046/j.1524-4725.1999.08145.x. PMID 9935087.Tier 3Supports: Uncontrolled open pilot investigation, n=25, Fitzpatrick skin types V and VI. Indications: 9 acne vulgaris, 5 post-inflammatory hyperpigmentation, 6 melasma, 5 rough/oily skin with enlarged pores. All patients were 'pre-treated for 2 weeks with hydroquinone 4% prior to undergoing a series of five salicylic acid chemical peels.' SA concentrations 20% and 30%, performed at 2-week intervals. Results: 'Moderate to significant improvement was observed in 88% of the patients. Minimal to mild side effects occurred in 16%.' SCOPE LIMITS: pilot study, no control or comparator arm, no blinded assessment, hydroquinone pre-treatment confounds the pigmentary outcomes, and the study cannot support a claim about Fitzpatrick I-IV since it enrolled only V-VI. This is the sole citation the International Peeling Society gives for 'safe in skin of all Fitzpatrick phototypes'. No funding or COI statement in the abstract record.
  3. Dainichi T, Ueda S, Imayama S, Furue M. Excellent clinical results with a new preparation for chemical peeling in acne: 30% salicylic acid in polyethylene glycol vehicle. Dermatol Surg. 2008 Jul;34(7):891-9; discussion 899. doi:10.1111/j.1524-4725.2008.34174.x. PMID 18363720.Tier 3Supports: Mixed animal-plus-human report. Verbatim from the abstract: 'We evaluated the effects of the preparation histologically in mice and its safety and efficacy in 44 volunteers with normally aged skin and in 436 patients with acne.' Results: 'Histologic studies in animals showed no inflammatory changes in the skin following topical application of SA-PEG. Volunteers noted an improved skin texture. In the acne patients, the comedones and papules disappeared, resulting in an excellent outcome. There was a notable absence of stinging and burning, edema, bleeding, or crusting in the treated area.' Rationale verbatim: 'Chemical peeling by salicylic acid in ethanol or another vehicle may be accompanied by stinging and burning followed by postinflammatory hyperpigmentation in the treated area, or salicylism.' Conclusion: 'The SA-PEG preparation appeared to be safe and effective, with minimal associated inflammation or adverse effects, even in Asian patients who tend to develop hyperpigmentation or keloids.' SCOPE LIMITS AND A CAUTION: this is the paper the International Peeling Society cites for a 'split-face study... showing superiority of 30% SA-PEG to 30% SA-HA', but the published abstract describes no randomisation, no split-face design, no control arm, no comparator concentration and no numerical effect size — outcomes are reported narratively ('excellent outcome'). Full text is paywalled (HTTP 402) and could not be verified. Population is Japanese/Asian. No funding statement available in the PubMed record.
  4. Ueda S, Mitsugi K, Ichige K, Yoshida K, Sakuma T, Ninomiya S, Sudou T. New formulation of chemical peeling agent: 30% salicylic acid in polyethylene glycol. Absorption and distribution of 14C-salicylic acid in polyethylene glycol applied topically to skin of hairless mice. J Dermatol Sci. 2002 Apr;28(3):211-8. doi:10.1016/s0923-1811(01)00168-2. PMID 11912008.Animal studyTier 3Supports: ANIMAL STUDY — male hairless mice, not humans. 14C-labelled 30% salicylic acid in PEG vehicle; ointment containing 3 mg salicylic acid in 10 mg vehicle applied to intact and to damaged skin. Verbatim results: 'In animals with intact skin, 1 h after application the plasma concentration of radioactivity was 1665.1 ng eq/ml, significantly lower than the 21437.6 ng eq/ml observed in mice with damaged skin' — i.e. roughly a 13-fold increase in systemic exposure when the barrier is broken. 'In the carcasses remaining after the treated intact and damaged skin had been removed, 0.09 and 11.38% of the applied radioactivity remained, respectively.' Authors' conclusion, verbatim: 'These findings confirm that 30% salicylic acid in PEG vehicle is little absorbed through the intact skin of hairless mice, and suggest that salicylism related to absorption through the skin of quantities of topically applied salicylic acid is not likely to occur in humans with intact skin during chemical peeling with this preparation.' SCOPE LIMIT: the extrapolation to humans is the authors' own inference from murine data; hairless mouse skin is more permeable than human skin. First author's affiliation is a private clinic (Ueda Setsuko Clinic, Fukuoka) and the same group developed the SA-PEG formulation. No funding statement in the PubMed record.Funding / interest: Author group (Ueda, with Dainichi) developed the 30% SA-PEG peel formulation being evaluated; no explicit funding statement in the PubMed record.
  5. Chen X, Wang S, Yang M, Li L. Chemical peels for acne vulgaris: a systematic review of randomised controlled trials. BMJ Open. 2018 Apr 28;8(4):e019607. doi:10.1136/bmjopen-2017-019607. PMID 29705755.Tier 1Supports: Systematic review using 'Standard Cochrane methodological procedures' — but published in BMJ Open, NOT a Cochrane Library review. Searched MEDLINE, CENTRAL and EMBASE via OvidSP through April 2017. Twelve RCTs, 387 participants. Verbatim results relevant to SA: 'trichloroacetic acid versus salicylic acid (SA) (percentage of total improvement: risk ratio (RR) 0.89; 95% CI 0.73 to 1.10)'; 'SA versus pyruvic acid (excellent or good improvement: RR 1.11; 95% CI 0.73 to 1.69)'; 'GA versus SA (good or fair improvement: RR 1.00; 95% CI 0.85 to 1.18)'; 'lipohydroxy acid versus SA (reduction of non-inflammatory lesions: 55.6% vs 48.5%, p=0.878)'; 'Combined SA and mandelic acid peeling was superior to GA peeling (percentage of improvement in total acne score: 85.3% vs 68.5%, p<0.001)'; 'SA peeling may be superior to JS peeling for comedones (reduction of comedones: 53.4% vs 26.3%, p=0.001) but less effective than phototherapy for pustules (number of pustules: MD -7.00; 95% CI -10.84 to -3.16).' Limitations verbatim: 'The methodological quality of the included RCTs was very low to moderate. Meta-analysis was not possible due to the significant clinical heterogeneity across studies.' Conclusion verbatim: 'Commonly used chemical peels appear to be similarly effective for mild-to-moderate acne vulgaris and well tolerated. However, based on current limited evidence, a robust conclusion cannot be drawn regarding any definitive superiority or equality among the currently used chemical peels.' Scope: mild-to-moderate acne only. Competing interests: 'None declared.'
  6. Kessler E, Flanagan K, Chia C, Rogers C, Glaser DA. Comparison of alpha- and beta-hydroxy acid chemical peels in the treatment of mild to moderately severe facial acne vulgaris. Dermatol Surg. 2008 Jan;34(1):45-50; discussion 51. doi:10.1111/j.1524-4725.2007.34007.x. PMID 18053051.Tier 2Supports: Split-face, double-blind, randomised controlled study, n=20, mild to moderately severe facial acne vulgaris. 30% glycolic acid (AHA) to one half-face versus 30% salicylic acid (BHA) contralaterally, every 2 weeks for six treatments; blinded evaluator counted papules and pustules. Verbatim results: 'Both chemical peels were significantly effective by the second treatment (p<.05) and there were no significant differences in effectiveness between the two peels. At 2 months posttreatment, the salicylic acid peel had sustained effectiveness. More adverse events were reported with the glycolic acid peel after the initial treatment.' Conclusion verbatim: 'The glycolic acid and salicylic acid peels were similarly effective. The salicylic acid peel had sustained effectiveness and fewer side effects.' SCOPE LIMITS: n=20 is small; no effect sizes or confidence intervals in the abstract; skin phototypes of participants not stated in the abstract; single US centre. No funding or COI statement in the PubMed record.
  7. Fung W, Orak D, Re TA, Haughey DB. Relative bioavailability of salicylic acid following dermal application of a 30% salicylic acid skin peel preparation. J Pharm Sci. 2008 Mar;97(3):1325-8. doi:10.1002/jps.21109. PMID 17694544.Tier 3Supports: Single-centre, single-sequence, two-period crossover pharmacokinetic study in nine healthy male and female subjects. Compared systemic exposure after FACIAL application of a 30% SA cosmetic skin peel formulation for 5 minutes versus an oral 650 mg aspirin dose. Verbatim: 'The mean (SD) maximum SA concentration (Cmax) was 0.81 (0.32) microg/mL and 56.4 (14.2) microg/mL. The AUC-based safety margin ratio was 50:1. A depot effect was observed during topical application of the skin peel solution as the absorption of SA continued beyond the 5-min application period. Plasma SA Cmax values were achieved from 1.4 to 3.5 h after topical application and from 0.5 to 1.5 h after oral aspirin. The plasma concentrations in the present study (30%; 5 min) were similar to that of a low concentration (2%) applied in a leave-on product to the same body surface area. In conclusion, our results suggest that the use of this SA facial peel should not pose any significant systemic health risks.' SCOPE LIMITS: n=9, healthy subjects with intact skin, FACE ONLY, single 5-minute application — it says nothing about repeated peeling, body peeling, or compromised barrier. The 'depot effect' finding is clinically important: absorption continued after the peel was removed.Funding / interest: Authors affiliated to L'Oreal USA, Clark, New Jersey — the study evaluates a cosmetic skin peel formulation and was conducted by employees of a cosmetics manufacturer.
  8. Arif T. Salicylic acid as a peeling agent: a comprehensive review. Clin Cosmet Investig Dermatol. 2015 Aug 26;8:455-461. doi:10.2147/CCID.S84765. PMID 26347269; PMCID PMC4554394.Tier 4Supports: Single-author narrative review, open access. Verbatim: SA is 'a lipid-soluble agent' that is 'miscible with epidermal lipids and sebaceous gland lipids in hair follicles'; it 'has keratolytic and comedolytic properties, although the exact mechanisms involved are not clear.' Anti-inflammatory dose range verbatim: 'The concentration at which the anti-inflammatory action of SA is most pronounced is between 0.5% and 5% (w/w)' — note this is far below peel strengths of 20-30%. Neutralisation verbatim: 'The SA peel has an advantage over the alpha-hydroxy acid peel in that the former does not need to be neutralized and the frost is visible once the peel is complete.' Frosting verbatim: 'Within 30 seconds to 1 minute of peeling, a white precipitate is formed, as a result of crystallization of the SA' and 'the frost seen in a SA peel represents precipitated SA, while the frost in a trichloroacetic acid peel is formed by precipitation of skin proteins.' Recovery: 'exfoliation cleared in 7-10 days. There were no cases of scarring or persistent post-inflammatory dyschromias.' Skin type: 'SA peels have been found to be safe and well tolerated by all racial/ethnic groups and in all skin types (Fitzpatrick I-VI).' Toxicity verbatim: 'Systemic toxicity due to cutaneous absorption is a very rare phenomenon, but is a serious concern' and 'In dermatological practice, salicylate toxicity been reported when 20% SA is applied to 50% of the body surface area. This has been mentioned with the use of 40% and 50% SA paste preparations.' Pregnancy verbatim: 'SA is classified by the US Food and Drug Administration as a pregnancy category C drug' and 'Use of SA peels is not recommended during pregnancy because the structure of SA is closely related to that of aspirin.' SCOPE LIMITS: narrative review, no systematic search, no evidence grading; the FDA pregnancy category system was retired in 2015 under the Pregnancy and Lactation Labeling Rule and carries no UK legal weight. Disclosure verbatim: 'The author reports no conflicts of interest in this work.' No funding statement given.
  9. Scientific Advisory Group on Chemical Safety of Non-Food and Non-Medicinal Consumer Products (SAG-CS). Opinion on Salicylic Acid Restrictions in Cosmetic Products. UK Office for Product Safety and Standards, January 2022 (discussed at SAG-CS meeting 21 July 2021).Tier 1Supports: Official UK regulatory advisory opinion; full PDF text extracted and read. Verbatim: salicylic acid (INCI: Salicylic Acid, CAS 69-72-7) 'is currently regulated under entry 98 of Annex III and entry 3 of Annex V of the UK Cosmetic Regulation No 1223/2009 (as amended)'. Annex III entry 98 table verbatim: 'Only allowed, when used for purposes other than preservative in: 1. Rinse off hair-products. 2. Other products with the exception of body lotion, eye shadow, mascara, eyeliner, lipstick and roll-on deodorant' at maximum concentrations '1. 3% / 2. 2%', with conditions 'Not to be used in applications that may lead to exposure of the end-user's lungs by inhalation', 'Not to be used in products for children under 3 years of age', 'Not to be used in oral products'. Annex V entry 3 (preservative use): '0.5 % (as acid)'. CLP status verbatim: 'In October 2018, as part of the 13th Adaptation to Technical Progress (ATP), salicylic acid under Commission Delegated Regulation (EU) 2018/1480 was classified as a Category 2 reproductive toxicant under EC Regulation No 1272/2008'. SCCS reference: 'In December 2018, the SCCS adopted an opinion... and concluded that salicylic acid is safe for consumer use when used as a preservative in cosmetic products at a concentration of up to 0.5%'. SAG-CS conclusions verbatim: 'Members agreed that there would be no appreciable increase in health risk following the addition of salicylic acid for purposes other than preservative function in body lotion, eye shadow, mascara, eyeliner, lipstick, and roll-on deodorant applications in a concentration of up to 0.5%. Members stated that salicylic acid use in products may need to be further reviewed with respect to body burden and aggregate exposure from other routes. Members noted that there may be a need to further consider the genotoxicity data in the future.' SCOPE LIMIT: this governs cosmetic products placed on the GB market. It does not address professional-use 20-30% peel solutions, does not license any procedure, and does not name chemical peeling.
  10. Bozzo P, Chua-Gocheco A, Einarson A. Safety of skin care products during pregnancy. Can Fam Physician. 2011 Jun;57(6):665-667.Tier 4Supports: Motherisk Update column (narrative Q&A format, not a systematic review), Motherisk Program, Hospital for Sick Children, Toronto. On salicylic acid absorption, verbatim: 'Undetectable from up to 25% in normal skin (depends on vehicle, pH, strength, and quantity applied). Levels might be higher when applied to damaged skin.' On pregnancy, verbatim: 'No studies have been conducted in pregnancy on topical use; however, as such a relatively small proportion is absorbed through the skin, it is unlikely to pose any risk to a developing baby.' SCOPE LIMITS: expert-opinion column, Canadian, no concentration threshold given, and it explicitly confirms the absence of pregnancy studies rather than providing evidence of safety. Competing interests: 'None declared.' No funding declared.

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Skinipedia is written by the educators at MSTA, the medic-led skincare training academy in Liverpool.