The four acids in professional use do different jobs, and strength is the last thing to choose between them. Glycolic has the smallest molecule and goes deepest, so it suits texture and lines on resilient skin. Lactic is larger and holds water, so it suits dullness and dehydration. Salicylic is oil-soluble and clears into the follicle, so it suits congestion. Mandelic is the largest and slowest, which makes it the one to reach for when pigmentation risk is a concern.
Key takeaways
- Molecule size sets penetration. Glycolic is the smallest and fastest, mandelic the largest and slowest.
- Glycolic: texture, fine lines and sun damage on an intact barrier. GlyPro at Level 5 and Level 6.
- Lactic: dullness and dehydration, humectant so less drying. Hydra-Bright AHA Glow Peel at Level 4.
- Salicylic with mandelic: congestion where pigmentation is also a risk. Beta-Clarity AHA Clarifying Peel at Level 4.
- Reactive or first-time skin starts on Flower Enzyme at Level 3, before any acid is considered.
Clients ask which acid is strongest. It is the wrong question, and answering it as asked leads to the wrong prescription more often than anything else in a skincare consultation.
The four acids in common professional use are not four points on one scale. They are four different tools, distinguished by molecule size, solubility and the part of the skin they reach. Choosing between them is a question about the skin in front of you, not about intensity.
Glycolic acid
The smallest molecule of the alpha hydroxy acids, which is why it penetrates fastest and furthest. That makes it the most capable of the four on texture, fine lines and accumulated sun damage, and the least forgiving on a compromised barrier.
In the Glo range it anchors the GlyPro line: GlyPro AHA Resurfacing Peel at Level 5 with 30% glycolic acid, for experienced peel clients working on texture and sun damage, and GlyPro Power Peel+ at Level 6 with 60% glycolic acid, the highest professional-grade peel in the range. In home care it appears in GlyPro Retinol Drops alongside retinol and gallic acid, and in Renew Serum alongside retinol and salicylic acid.
Reference entry: glycolic acid.
Lactic acid
A larger molecule than glycolic, so it works more gradually, and a humectant in its own right, so it holds water in the skin rather than drawing it out. That combination makes it the acid for dullness, dehydration and uneven tone, and the more comfortable choice for a client who wants results without a difficult recovery.
In the range: Hydra-Bright AHA Cleanser with vegan lactic acid for daily use, and Hydra-Bright AHA Glow Peel at Level 4 with 20% vegan lactic acid, which suits dull skin, pigmentation and a seasonal refresh across most skin types. Lactic also appears in Hydra-Bright Alpha Arbutin Drops and in the BHA Clarifying Gel Moisturizer, where it is paired with salicylic acid.
Reference entry: lactic acid.
Salicylic acid
The one beta hydroxy acid of the four, and the only one that is oil-soluble. That is the whole point of it: it moves through sebum and clears into the follicle, where the AHAs cannot go. It suits oily and congested skin, and it suits it in a way no alpha hydroxy acid matches.
In the range: Beta-Clarity Clear Complexion Pads and Beta-Clarity Pro 5 Liquid Exfoliant for regular use, Renew Serum where it sits alongside retinol and glycolic acid, BHA Clarifying Gel Moisturizer where it is paired with lactic acid, and in-clinic within the Beta-Clarity AHA Clarifying Peel.
Reference entry: salicylic acid.
Mandelic acid
The largest molecule of the alpha hydroxy acids, derived from bitter almonds, and the slowest to penetrate. Slow is the feature. A gradual, even action across the skin surface is what makes mandelic the acid to consider when the concern is congestion but the skin also carries a pigmentation risk that a faster acid would provoke.
In the range it pairs with salicylic acid in Beta-Clarity AHA Clarifying Peel at Level 4, 20% mandelic with salicylic, for oily and blemish-prone skin and post-blemish marks.
Reference entry: mandelic acid.
Before the acids: the enzyme option
Not every skin should meet an acid first. Flower Enzyme at Level 3 works through hibiscus flower extract, pumpkin ferment and ficin rather than hydroxy acids, gently lifting superficial dead cells while the flower extracts contribute mild organic acids and antioxidant support. It suits most skin types other than extremely sensitive, and it is the right opening move for a reactive skin or a first-time peel client whatever their eventual acid profile turns out to be.
The home-care tier
Two liquid exfoliants carry the same logic into the client's routine at a frequency they can manage themselves.
Hydra-Bright Pro 5 Liquid Exfoliant is a 10% AHA and PHA blend, clinically proven to boost radiance after one use, with 88% of users reporting brighter skin after one week. The brightening route.
Beta-Clarity Pro 5 Liquid Exfoliant is a 10% AHA and BHA blend, with 90% of users reporting improved skin texture after one week. The congestion route.
Both are the bridge between appointments, and both are why the acid conversation in the consultation has a retail outcome rather than only a clinical one.
Mapping the acid to the ladder
The acid choice and the peel level are two decisions, not one. A client who suits mandelic and a client who suits glycolic can both be appropriate at Level 4, and neither is more advanced than the other.
Where the levels do matter is progression and channel. Levels 1 and 2 are the at-home kits, formulated for safe unsupervised use. Levels 3 to 6 are professional-exclusive back bar and never appear in a retail recommendation. A first-time peel client starts at Level 3 on Flower Enzyme, whatever their acid profile suggests, so the response can be read before an acid is committed to. The full ladder is set out in which peel level.
The question to ask instead
Not "how strong can this skin take". Ask what the skin is doing, where the problem sits in it, and how much recovery this client can accommodate in the fortnight ahead. The acid follows from the answer, and the level follows from experience and tolerance.
Strength was never the variable. Fit was.
FAQs
What is the difference between glycolic, lactic, salicylic and mandelic acid?
Molecule size and solubility. Glycolic is the smallest alpha hydroxy acid, so it penetrates fastest and suits texture and fine lines on an intact barrier. Lactic is larger and holds water, so it suits dullness and dehydration with a gentler recovery. Salicylic is oil-soluble and clears into the follicle, so it suits congestion. Mandelic is the largest and slowest, useful where pigmentation risk rules out a faster acid.
Which acid suits congested or blemish-prone skin?
Salicylic acid, because it is oil-soluble and moves through sebum into the follicle where the alpha hydroxy acids cannot reach. In the Glo range it runs through the Beta-Clarity Clear Complexion Pads, the Beta-Clarity Pro 5 Liquid Exfoliant, Renew Serum and the BHA Clarifying Gel Moisturizer. In clinic it pairs with mandelic acid in the Beta-Clarity AHA Clarifying Peel at Level 4, which suits oily, blemish-prone skin and post-blemish marks.
Which acid is strongest?
Strength is the wrong comparison. Glycolic penetrates fastest because it has the smallest molecule, but a 20% lactic peel and a 20% mandelic peel both sit at Level 4 and neither is more advanced than the other. The right question is what the skin is doing, where in the skin the concern sits, and how much recovery time the client can accommodate. The acid follows from that.
The full peel protocols are in the Glo Pro protocol library, and the range sits in the peel collection. Questions on selection, call 01636 402521 or email pro@gloskin.beauty.