Unit 2 · Alpha-hydroxy acids
Same acid family, three molecules that behave nothing alike
Glycolic, lactic and mandelic acid share a mechanism but differ enormously in molecular size, penetration and tolerability. By the end you should be able to match each of the three major AHAs — and choose between them — based on skin type, condition and Fitzpatrick phototype.
Learn · The AHA family
What defines an alpha-hydroxy acid
AHAs are a family of water-soluble organic acids characterised by a hydroxyl group at the alpha carbon adjacent to the carboxylic acid group. They are derived from natural sources — fruit, milk, sugar cane — though most cosmetic-grade AHAs are synthetically produced.
All AHAs share a common mechanism: lowering surface pH to accelerate corneocyte shedding. What they do not share is molecular size, and molecular size is what separates a potent, deeply penetrating AHA from a gentle, surface-acting one.
Learn · Glycolic acid
Glycolic acid — the smallest, deepest-penetrating AHA
- Source
- Sugar cane
- Molecular weight
- 76 Da — the smallest AHA
- Typical OTC range
- 2–10%
- Professional peel range
- 20–70%
- Penetration depth
- Greatest of all AHAs
Glycolic acid is the most studied and clinically validated AHA. Its small molecular size gives it the deepest epidermal penetration of any AHA — reaching the stratum spinosum and papillary dermis at higher concentrations. This makes it the most potent but also the most potentially irritating member of the AHA family.
Beyond exfoliation, glycolic acid has demonstrated ability to stimulate collagen I and III synthesis by fibroblasts, normalise aberrant corneocyte turnover, reduce comedonal congestion and improve post-acne hyperpigmentation with regular use.
Photoageing, fine lines, dull texture: first-line AHA. Greatest penetration means greatest collagen stimulation and surface renewal, with best results at 8–10% daily-use concentrations over 12+ weeks of consistent use.
Acne-prone, comedonal skin: effective at normalising follicular keratinisation, but less effective than salicylic acid because it is water-soluble and does not penetrate the sebaceous follicle. Best used alongside a BHA in acne management.
Sensitive skin, Fitzpatrick IV–VI: caution. The smallest molecule carries the highest irritation potential and can trigger post-inflammatory hyperpigmentation in darker skin types if used at too high a concentration or frequency. Lactic or mandelic acid may be the better primary choice for darker phototypes.
Glycolic acid is the most potent of the common AHAs because:
Select an option to commit. The reasoning appears afterwards.
At 76 Da, glycolic acid is the smallest common AHA molecule, which gives it the deepest epidermal penetration of the family — reaching the stratum spinosum and papillary dermis at higher concentrations.
That same property cuts both ways clinically: the depth that produces the strongest collagen stimulation and surface renewal is also what makes glycolic acid the most potentially irritating AHA, and the one to use with the most caution in sensitive or darker skin.
Learn · Lactic acid
Lactic acid — gentler penetration, built-in hydration
- Source
- Fermented milk (sour milk, yogurt)
- Molecular weight
- 90 Da
- Typical OTC range
- 2–10%
- Professional peel range
- 20–50%
- Additional property
- Humectant — attracts and retains moisture
Lactic acid's larger molecular size compared to glycolic acid translates to slower, gentler penetration — more appropriate for sensitive skin while retaining meaningful exfoliating and collagen-stimulating efficacy. Its unique additional property is a humectant effect: it attracts and retains water in the stratum corneum, making it uniquely beneficial for dry and dehydrated skin.
Lactic acid is also a natural component of the skin's own natural moisturising factor (NMF) — it contributes to the skin's own hydration and acid mantle. This biological familiarity is part of why it is generally very well tolerated.
Dry, dehydrated skin: the preferred AHA. The humectant property means it exfoliates and hydrates simultaneously — addressing two common concerns in dry and mature skin at once.
Sensitive skin: gentler than glycolic at equivalent concentrations. Often the AHA of choice for patients transitioning from no exfoliation, or who have shown intolerance to glycolic acid.
Fitzpatrick III–V: lower irritation risk than glycolic at equivalent percentages — a safer starting AHA for medium-toned skin with some pigmentation concern.
A patient with dry, dehydrated, sensitive skin wants to introduce an AHA without stripping moisture from an already compromised barrier. The AHA to recommend first is:
Select an option to commit. The reasoning appears afterwards.
Lactic acid's larger molecular size gives it slower, gentler penetration than glycolic acid, and its humectant property means it attracts and retains water in the stratum corneum. For a dry, dehydrated, sensitive patient, it exfoliates and hydrates at the same time.
This is also why lactic acid is a natural component of the skin's own natural moisturising factor — the biological familiarity is part of why it is so well tolerated, and why it is often the AHA of choice for a patient who has struggled with glycolic acid.
Predict · Acid selection by phototype
Photoageing looks the same in two patients — the right AHA might not
Two patients present with the same concern — photoageing and dull texture — and both are candidates for regular AHA use. One is Fitzpatrick II, the other Fitzpatrick V. Before reading on, decide whether the same AHA is the right recommendation for both.
Same concern, same goal — does the same AHA suit a Fitzpatrick II patient and a Fitzpatrick V patient equally well?
Hold your answer before you open this. The value is in having committed to a position on phototype and acid choice first.
No. Glycolic acid — the deepest-penetrating, most potentially irritating AHA — carries a real risk of triggering post-inflammatory hyperpigmentation in darker skin if used at too high a concentration or frequency, so it is a reasonable first choice for the Fitzpatrick II patient but a cautious one for the Fitzpatrick V patient.
Lactic acid carries a lower irritation risk than glycolic at equivalent percentages and is positioned as a safer starting AHA for medium-toned skin with pigmentation concern. For the Fitzpatrick V patient, lactic — or, as the next section covers, mandelic — is the more defensible starting point.
Learn · Mandelic acid
Mandelic acid — the gentlest AHA, with antibacterial reach
- Source
- Bitter almonds (amygdalin hydrolysis)
- Molecular weight
- 152 Da — the largest common AHA
- Typical OTC range
- 5–10%
- Professional peel range
- 20–40%
- Additional properties
- Antibacterial, anti-inflammatory
Mandelic acid's large molecular size means it penetrates the skin more slowly than glycolic or lactic acid — making it the gentlest and most suitable AHA for sensitive and darker skin types.
Beyond exfoliation, mandelic acid has demonstrated direct antibacterial activity against Cutibacterium acnes (formerly Propionibacterium acnes) and Staphylococcus aureus — a secondary benefit in acne management that distinguishes it from other AHAs.
Fitzpatrick IV–VI: the preferred AHA for professional peels and home-use exfoliants in darker phototypes. Lower irritation risk, direct antibacterial activity and anti-inflammatory properties reduce PIH risk significantly compared with glycolic acid at equivalent strengths.
Acne and congestion in darker skin: the combination of exfoliation and direct anti-acne activity, without the PIH risk of glycolic, makes mandelic particularly valuable in this demographic — often combined with azelaic acid.
Sensitive skin: an excellent choice where glycolic acid is too stimulating. Slower penetration provides a more gradual, tolerable introduction to acid exfoliation.
For professional peels in Fitzpatrick IV–VI skin, which acid offers the best balance of efficacy and safety?
Select an option to commit. The reasoning appears afterwards.
Mandelic acid's large molecular size — the largest of the common AHAs — gives it the slowest penetration and the lowest irritation potential. Its direct antibacterial and anti-inflammatory properties add a second layer of PIH-risk reduction that no other AHA in this module offers.
This is why mandelic acid is positioned as the preferred AHA for professional peels in Fitzpatrick IV–VI skin — not because it is the strongest, but because it is the one that balances meaningful efficacy against the specific risk that matters most in darker phototypes.
Learn · Other AHAs
Citric, malic and tartaric acid
Citric acid (from citrus fruit), malic acid (from apples) and tartaric acid (from grapes) are less frequently used as standalone exfoliants. They appear most often in combination AHA formulations and as buffering agents to adjust pH. Citric acid also has chelating and antioxidant properties. At typical use concentrations in combination products, their individual exfoliating contribution is modest.
| AHA | MW (Da) | Penetration | Best skin type | Special property |
|---|---|---|---|---|
| Glycolic | 76 | Deepest | Normal / oily / photoaged | Strongest collagen stimulation; most studied |
| Lactic | 90 | Moderate | Dry / sensitive / mature | Also a humectant; NMF component |
| Mandelic | 152 | Slowest | Darker phototypes / sensitive / acne | Antibacterial; anti-inflammatory; safest for Fitzpatrick IV–VI |
| Citric / malic / tartaric | Variable | Moderate | Combination formulas | Primarily pH buffering and antioxidant in combination products |
Unit 2 summary
Clinical takeaways
- Molecular size, not strength alone, separates the three major AHAs. Glycolic (76 Da) penetrates deepest and is the most potent and most irritating; lactic (90 Da) is moderate and adds humectant benefit; mandelic (152 Da) is the largest, slowest and gentlest.
- Glycolic acid's greatest strength is also its greatest risk. Deep penetration drives the strongest collagen stimulation but also the highest PIH risk in Fitzpatrick IV–VI skin — caution and lower starting concentrations apply.
- Lactic acid solves two problems in dry, dehydrated skin at once. Its humectant property means it exfoliates and hydrates simultaneously, and it is generally well tolerated because it mirrors a component of the skin's own natural moisturising factor.
- Mandelic acid is the preferred AHA for darker phototypes. Its slow penetration, antibacterial activity and anti-inflammatory properties combine to lower PIH risk more than any other single AHA in professional peel use.