Exfoliating acids at home: AHA, BHA and PHA, which one, how often and what not to do

Each family of acids works in a different place in the skin. AHAs are water-soluble and act at the surface, breaking the bonds between dead cells. BHA — salicylic acid — is oil-soluble and gets into the follicle, where the sebum is. PHAs are larger molecules, they penetrate less and so they irritate less. Choosing well is not choosing the strongest: it is choosing the one that reaches where the problem is, which is why they are not three versions of the same thing at different strengths.

And there is one fact no website in this sector publishes and that is worth having before buying: in Europe the only one of the three with a concentration limit written into law is salicylic acid. Glycolic, lactic and mandelic acid do not appear in the lists of restricted substances in the Cosmetics Regulation. It is exactly the opposite of what almost everyone assumes.

This page is about chemical exfoliation at home. The peel done in clinic — different concentrations, different control, different risk — is a separate medical treatment and has its own page.

The three families, and what each one does

AHAs — alpha hydroxy acids

Glycolic, lactic, mandelic, and also malic, tartaric and citric. They are water-soluble: they stay at the surface and act by dissolving the cohesion between corneocytes, the dead cells of the stratum corneum. The result is a smoother, more reflective surface and, with continued use, an improvement in fine texture.

• Glycolic is the smallest molecule in the family and the one that penetrates most; also the one that irritates most.
• Lactic is somewhat larger, penetrates less and also retains water, so it is the option where there is dryness as well.
• Mandelic is the largest of the three, penetrates slowly and is the best tolerated; it is often used on skin prone to hyperpigmentation because of its lower risk of inflammation.

What they are for at home: uneven texture, a dull appearance, superficial pigmentation, fine lines.

BHA — beta hydroxy acid: salicylic acid

It is oil-soluble, and that is the whole difference: it mixes with sebum and gets into the follicle, where no AHA reaches. That is why it is the one of choice when the problem is pores and oily skin, and why it is in so many cleansers and toners for acne-prone skin.

🚨 It is also the only one of the three with an express legal restriction in Europe, and the next section sets that out.

What it is for at home: visible pores, congestion, oily skin, a tendency to blackheads.

PHAs — polyhydroxy acids

Gluconolactone, lactobionic acid. Chemically they are relatives of the AHAs with several more hydroxyl groups, which makes them notably larger molecules. They penetrate more slowly and more superficially, which is why they are the sensible option in sensitive, reactive or rosacea-prone skin, where a glycolic is harsh. They are also hygroscopic: they retain water.

There is recent clinical evidence in that direction. A 2025 study compared, in 200 people with mild rosacea erythema, a topical serum with 5% gluconolactone used for 30 consecutive days against a single session of intense pulsed light: both groups improved in erythema, overall facial appearance and several physiological parameters of the skin. ⚠️ It is a paired-control study, not a randomised trial, and it compares very different things — a month of home use against a single session; it is cited as a signal that a PHA is a reasonable tool in reactive skin, not as proof that it replaces treatment with pulsed light.

What they are for at home: sensitive skin, redness, anyone who tolerates neither AHAs nor BHA.

The legal limit, which is where the surprise is

This is the part that appears on no Spanish skincare blog and that can be checked against the official text in ten minutes.

Salicylic acid is in Annex III

Annex III of Regulation (EC) 1223/2009 is the list of substances a cosmetic may not contain except under restrictions. Salicylic acid is entry 98, and its conditions are these:

Type of product	Maximum concentration
Rinse-off hair products	3.0%
Other products (including leave-on facial products)	2.0%
Body lotions, eye shadows, mascaras, eyeliners, lipsticks and roll-on deodorants	0.5%

And three conditions that are read less and weigh just as much:

• 🚨 “Not to be used in preparations for children under 3 years of age”, with that warning obligatory on the label.
• Not to be used in applications that may lead to exposure of the lungs by inhalation, or in oral products.
• 🔑 “These levels include any use of salicylic acid.” That is the decisive sentence: salicylic acid is also authorised as a preservative, and the limit is not for the active, it is for the total. A product cannot carry 2% as an exfoliant and add more as a preservative.

The AHAs are not there

Glycolic, lactic and mandelic acid appear neither in Annex II (prohibited substances) nor in Annex III (restricted substances) of the consolidated text of the Regulation. They have no specific concentration limit in European cosmetics.

⚠️ This does not mean anything goes, and it is worth saying precisely. The Regulation requires in any case that the product be safe under normal conditions of use, with a signed safety assessment and a responsible person standing behind it. What there is not, is a number written into an annex.

🔑 The paradox, in short: the acid people associate with acne and with the pharmacy — salicylic — is the one the European Union limits to 2% on the face. The ones sold at 10%, 20% and 30% in a bottle have no limit of their own. Anyone comparing products by the number on the packaging is comparing things the law does not measure in the same way.

What the evidence actually measures (and on whom)

Here we have to be honest, because this is where a commercial page normally says nothing.

Almost all the clinical literature on acids is about professional peels, not home use. A recent and representative example: a 2025 study in 180 women in two age bands measured hydration and elasticity after a series of five exfoliating treatments with 40% mandelic or lactobionic acid, applied in clinic, with assessment 14 days after the last session. At 40% and over five sessions. That is not what is in the bottle you buy for home, and transferring its results to a 5% toner would be exactly the kind of leap this cluster avoids.

The review that has addressed the underlying question directly — do AHAs benefit or harm? — concludes that the answer depends on the concentration: at low concentrations they improve the skin, at high concentrations they can damage it, and that same review notes that it remains unclear whether AHAs increase or reduce photodamage. It also warns of adverse reactions described in users: swelling, stinging and itching.

And a 2024 clinical and regulatory review closes in the same direction: AHAs improve texture and radiance and are useful in acne, hyperpigmentation and photoageing, but the optimal concentrations for achieving efficacy with minimal side effects are still not precisely defined, and it stresses regulatory compliance — concentration and pH — as a condition of safety.

🔑 What can be stated without exaggerating: a well-chosen, well-dosed acid improves texture, radiance and pores with continued use. What cannot be stated is a percentage of improvement, or that a bottle at home does what a session in clinic does.

How often, and how to introduce them

There is no dosage because a cosmetic does not have one. What there is, is a sensible way to start, the same stepped logic as with retinol:

Phase	Frequency	How long it lasts
Start	Once a week, at night	2-3 weeks
If there is no discomfort	Twice a week	3–4 weeks
Usual maintenance	2–3 times a week as the ceiling for most people	indefinitely

• One acid at a time. You do not alternate glycolic and salicylic in the same week to “cover more”.
• At night. And with sunscreen during the day, without exception.
• If the skin feels tight, stings or has an odd shine, space it out, do not compensate with more moisturiser.
• A mechanical exfoliant — a scrub — is not added to an acid. It is the classic mistake: people think one prepares the skin for the other, and what it does is double the assault.

⚠️ And one warning worth more than the table: most people do not need to exfoliate three times a week. Twice is enough for almost everyone, and once for sensitive skin.

What not to do

• 🚨 Do not mix acid and retinol on the same night while you are building tolerance. Alternate nights.
• 🚨 Do not use acids on irritated, flaking or eczematous skin. If the skin is already broken, exfoliating breaks it further. What is needed is repair: the skin barrier and sensitive skin.
• 🚨 Do not apply them on the same day as a laser, pulsed light or a peel, or on irritated skin before surgery. How long beforehand to stop them has its own section below, and the answer is surprising.
• 🚨 Do not use them on a mark without knowing what it is. It is the warning that runs through this whole cluster: a pigmented lesion is diagnosed before it is treated. It is in creams for facial pigmentation.
• ⚠️ Do not apply around the eyes unless the product is formulated for that area: there the skin is around 573 microns thick and tolerance is different (eye creams).
• ⚠️ Do not add an exfoliating cleanser + an acid toner + an acid serum. It is the most common way of over-exfoliating without noticing, because each product on its own seems gentle.

How to tell you have overdone it

Over-exfoliated skin does not flake: it shines. It becomes tight and reactive, it stings with products it used to tolerate, it reddens easily and it soaks up moisturiser in seconds without stopping feeling tight. It is often interpreted as “I have become sensitive” and answered by buying more product, which is exactly the opposite of what is needed.

🔑 The treatment is taking away, not adding: all acids are stopped for two to four weeks, leaving a gentle cleanser, a moisturiser and sunscreen, and the barrier recovers on its own.

How long before a laser, a peel or surgery do you have to stop them?

It is the practical question of this page, and the honest answer starts with something uncomfortable:

🔑 The interval everyone repeats — “three days”, “a week” — has no trials behind it. The most recent systematic review of skincare around aesthetic procedures brought together 104 publications and found that what is published on pre-procedure care rests on a level of evidence ≤ 3, with very few trials comparing a specific regimen against doing nothing. The numbers in circulation are custom, not results.

And there is one finding that runs directly counter to the fear. The underlying worry is that an active leaves the skin “thin” and delays healing. It was tested in a randomised arm-against-arm trial: three weeks of tretinoin on one forearm and placebo on the other, then CO₂ laser on both.

⚠️ There was no difference at all: not in re-epithelialisation, not in collagen, not in how quickly the redness settled. The study was looking at whether tretinoin improved the result and concluded that it did not — but in passing it made clear that it does not make it worse either.

So what can be said with a basis? This:

Situation	What the evidence supports
Peel in darker skin types (IV–VI)	Stop retinoids 1–2 weeks beforehand. And the reason is not healing: it is avoiding pigmentation and scarring
Peel in lighter skin types	Low-dose tretinoin can be used beforehand to improve the result, not merely tolerated
Laser	There is no established interval. The expert panel itself leaves it to clinical judgement
Surgery	It is for the team operating on you to decide

🔑 And from that comes the criterion that does work, which is not a date: what decides is the state of your skin on the day. Skin that is irritated, red or flaking is a reason to postpone even if you stopped ten days ago. Calm skin does not become dangerous because you used an acid on Tuesday.

🚨 With one exception that is a date: if you have a darker skin type and you are having a peel, stop retinoids one or two weeks beforehand. There the risk is pigmentation, and that one is documented.

The practical advice: take the packaging to the consultation. It is quicker than describing it, and whoever is going to treat you decides with the product in front of them — which is how this should always be decided.

Frequently asked questions

Which acid suits me?

It depends on the problem, not on a skin “type”. If what bothers you is texture, dull tone or a superficial mark, an AHA. If it is pores, congestion or oily skin, a BHA. If your skin is irritated by everything or there is redness, a PHA. And if there are several things at once, you start with one.

Is a more concentrated acid better?

Not necessarily, and the literature itself says so: with AHAs, the benefit or the harm depends on the concentration. The pH of the product and the contact time also count as much as the percentage. A well-formulated 5% used twice a week delivers more than a 20% abandoned because of irritation.

Do acids thin the skin?

It is a myth with a grain of truth. What they do is reduce the thickness of the stratum corneum, the layer of dead cells, while continued exposure to AHAs has been associated with an increase in the thickness of the living epidermis. What does thin and damage is excessive use, which breaks the barrier.

Can I use an acid if I have rosacea or very sensitive skin?

With caution and preferably a PHA, which is the family that penetrates least. But rosacea is a diagnosis and its treatment is not decided on a web page: it is assessed, and the light-based options are in the page on IPL for redness.

Does an acid at home replace a peel in clinic?

No. The concentrations, pH and contact times are of another order, with medical supervision and the ability to reach depths a cosmetic cannot. What a home acid does do is maintain the result between sessions. The comparison is in chemical peel in Valencia.

Do I have to stop acids before surgery?

Yes, but the interval is not as established as it seems: the numbers in circulation are custom and have no trials behind them. What decides is the state of your skin on the day of the procedure. The exception with backing is a peel in darker skin types, where retinoids are stopped one or two weeks beforehand because of the risk of pigmentation. Take the packaging to the consultation.

And the grain exfoliants, the scrubs?

They act by friction, not chemically. They are not worse by definition, but they are far harder to control — the pressure is applied by you — and they should not be added to an acid. If you are already using a chemical exfoliant, the mechanical one is redundant.

🔑 And there is a legal change that explains why the grain is not what it was a few years ago. Regulation (EU) 2023/2055 banned microbeads — microparticles of synthetic polymers used as an abrasive to exfoliate, polish or clean — from 17 October 2023, with no transitional period at all, while other rinse-off products were given until 2027 and lip and nail products until 2035. That is why the abrasive in a current exfoliant is mineral or plant-based — clays, silica, ground apricot kernel, jojoba waxes — and not plastic. ⚠️ It changes the environmental impact, not the advice: an irregular grain still scratches more than a spherical one, and the pressure is still applied by whoever uses it.

And enzyme exfoliants?

They use proteases — papain from papaya, bromelain from pineapple — which break down the proteins binding the dead cells, rather than dissolving that bond with acidity. In practice they are the gentlest of the three options and are usually well tolerated on reactive skin. ⚠️ The trade-off is that the published clinical evidence on home enzyme exfoliation is very thin, considerably less than for the acids, so there are no numbers to offer here: they are used for tolerability, not because they are better documented.

Sources

• Nikolis A, Nestor MS, Czuwara J, et al. Concomitant Use of Dermo-Cosmetic Skin Care in Aesthetic Procedures: Systematic Review with Expert Panel Recommendations. Clinical, Cosmetic and Investigational Dermatology, 2026;19:577128. PMID 41773167. — A systematic review of 104 publications (70 on energy-based devices, 25 on injectables and microneedling, 21 on peels and 10 on microdermabrasion) with an expert panel: pre-procedure care rests on a level of evidence ≤ 3 and there are very few trials comparing a specific regimen against not treating; the panel recommends stopping retinoids 1–2 weeks before a peel in skin types IV–VI to avoid dyschromia and scarring, while in lighter skin types low-dose tretinoin (0.05–0.1%) can be used before a peel to improve the result; no specific interval is established before laser, which is left to clinical judgement.
• Orringer JS, Kang S, Johnson TM, et al. Tretinoin treatment before carbon-dioxide laser resurfacing: a clinical and biochemical analysis. Journal of the American Academy of Dermatology, 2004;51(6):940-6. PMID 15583586. — A randomised within-patient trial: three weeks of tretinoin on one forearm and placebo on the other before CO₂ laser, with real-time PCR quantification of procollagen I and III and matrix metalloproteinases 1, 3 and 9, measurement of re-epithelialisation with a computer-generated template and colorimetry of the erythema. No substantial differences were found in biochemical markers or clinical variables: no greater collagen formation, no accelerated re-epithelialisation, no faster resolution of the erythema with pre-treatment.

• Regulation (EC) No 1223/2009 of the European Parliament and of the Council on cosmetic products — Annex III, entry 98 (“2-hydroxybenzoic acid / Salicylic acid”, CAS 69-72-7, EC 200-712-3). Consolidated text at EUR-Lex. — Maximum concentrations: (a) rinse-off hair products, 3.0%; (b) other products except body lotions, eye shadows, mascaras, eyeliners, lipsticks and roll-on deodorants, 2.0%; (c) those last ones, 0.5%. Other restrictions: “Not to be used in preparations for children under 3 years of age”; not to be used in applications that may lead to exposure of the lungs by inhalation; not to be used in oral products; for purposes other than inhibiting the growth of micro-organisms, with the presentation of the product having to make those other purposes evident; and “these levels include any use of salicylic acid”. Obligatory label warning: “Not to be used for children under 3 years of age”. Verified against the consolidated text on 28-08-2026. Confirmed in the same text that glycolic, lactic and mandelic acid do not appear in Annexes II or III.
• Commission Regulation (EU) 2023/2055 of 25 September 2023 amending Annex XVII to Regulation (EC) No 1907/2006 (REACH) as regards synthetic polymer microparticles. EUR-Lex (OJ L 238, 27-09-2023). — Paragraph 6(b) of entry 78 grants until 17 October 2027 for “rinse-off products” except those “containing synthetic polymer microparticles for use as an abrasive, that is to exfoliate, polish or clean (‘microbeads’)”; those were given no transitional period at all, and recital 21 explains why: the industry was expected to have phased out their use voluntarily by 2020. Other deadlines for comparison: encapsulation of fragrances, 2029; lip and nail products, 2035. The Regulation entered into force twenty days after publication. Verified against the EUR-Lex text on 28-08-2026.
• Tang SC, Yang JH. Dual Effects of Alpha-Hydroxy Acids on the Skin. Molecules, 2018. PMID 29642579. — A review of the biological effects of AHAs on human keratinocytes and in an animal model. It explicitly poses the question of whether AHA is a friend or an enemy of the skin and concludes that it depends on the concentration. It notes that it remains unclear whether AHAs increase or reduce photodamage, and warns of adverse reactions described in users of AHA products: swelling, stinging and itching.
• Evaluating the Efficacy and Safety of Alpha-Hydroxy Acids in Dermatological Practice: A Comprehensive Clinical and Legal Review. Clinical, Cosmetic and Investigational Dermatology, 2024. PMID 39050562. — A clinical and regulatory review. It describes AHAs as promoting skin renewal, collagen and elastin synthesis and improvement in texture and radiance, with utility in acne, hyperpigmentation and photoageing. 🔑 It states that the optimal concentrations for achieving therapeutic efficacy with minimal side effects are still not precisely defined and stresses regulatory compliance — concentrations and pH — as a condition of safety.
• Comparison of Efficacy and Willingness to a 5% Gluconolactone-Based Topical Serum and Intense Pulsed Light in Mild Erythema of Rosacea: A Paired Control Study. Journal of Cosmetic Dermatology, 2025. PMID 40111222. — 200 subjects with mild erythema: 100 used a serum with 5% gluconolactone for 30 consecutive days and 100 received a single session of intense pulsed light. Both groups improved in erythema (CEA), overall facial appearance (VAS) and physiological parameters (pores, tone, sebum). ⚠️ A paired-control, non-randomised study comparing interventions of very different nature and duration.
• Influence of Lactobionic Acid on Hydration and Elasticity Parameters in Women Aged 30-40 and 50-60 Years in Comparison to Mandelic Acid. Journal of Clinical Medicine, 2025. PMID 40095547. — 180 women in two age bands; in 120 of them a series of five exfoliating treatments with 40% mandelic or 40% lactobionic acid was applied, with measurement of hydration (Tewameter) and elasticity (cutometry) 14 days after the last session. 🔑 It is cited expressly as an example of the available evidence coming from professional peels and not from home use: at 40% and in clinic.

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The most frequent question about acids is not which one to buy: it is why the skin has got worse while using them. Almost always the answer is that it is being exfoliated three times more than necessary, with products that overlap without anyone noticing. Reviewing what you already have at home and removing what is redundant is free and usually enough; and if a peel in clinic is also needed, that is decided with the skin in front of us. You can see the products we work with in the exfoliants and acids shop.

Clinical confirmation by the surgeon

Last reviewed: 17 September 2026. This content is for information only and does not replace an individual assessment by a healthcare professional.