Lactic acid guide: the gentle AHA that exfoliates and hydrates simultaneously

Lactic acid is an alpha-hydroxy acid (AHA) derived from fermented milk — the first AHA documented in historical skincare (Cleopatra's milk baths are the canonical reference). In modern skincare it occupies a specific and well-evidenced niche: a chemical exfoliant gentle enough for dry, sensitive, and pregnancy-safe use, with the additional property of simultaneously humectating the skin. Here is the complete guide.

What makes lactic acid different from other AHAs

Molecular weight: larger = gentler penetration

| AHA | Molecular Weight | Relative Penetration Speed |
|---|---|---|
| Glycolic acid | 76 Da | Fastest — deepest penetration |
| Lactic acid | 90 Da | Moderate — controlled penetration |
| Mandelic acid | 152 Da | Slowest — most superficial |

Lactic acid at 90 Da is 18% larger than glycolic acid. This size difference translates directly to penetration speed: lactic acid diffuses through the stratum corneum more slowly than glycolic acid, producing a more controlled exfoliation with less irritation potential.

Clinical implication: At equivalent percentages and pH, lactic acid produces less erythema, less stinging, and less barrier disruption than glycolic acid — making it the preferred AHA for:
Dry or dehydrated skin
Sensitive or rosacea-prone skin
Skin new to chemical exfoliants
Pregnancy (discussed below)

The dual mechanism: exfoliation + humectancy

Lactic acid is unique among the AHAs because it is a natural moisturizing factor (NMF) component — it is naturally produced in skin as part of the amino acid/organic acid pool of humectant molecules that maintain stratum corneum water content.

Exfoliating mechanism (shared with all AHAs):
Reduces corneocyte cohesion by activating kallikrein serine proteases at low pH
Chelates calcium ions required for corneocyte adhesion
Accelerates desquamation → smoother texture, improved tone, reduced fine lines

Humectant mechanism (lactic acid-specific):
Applied lactic acid acts as an NMF component → binds water in the stratum corneum
Increases hydration in the skin's outer layers independently of its exfoliating activity
Studies have shown topical lactic acid increases ceramide synthesis — the lipid component of the skin barrier — improving barrier function over time

The practical result: where glycolic acid exfoliates, lactic acid exfoliates and hydrates simultaneously. A dry-skin patient using lactic acid does not experience the added dryness and dehydration that can accompany glycolic acid use.

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The evidence: barrier improvement and NMF function

Rawlings et al. 2004

*Rawlings AV, Davies A, Carlomusto M, et al. (2004). Effect of lactic acid isomers on skin barrier function, stratum corneum integrity and appearance of photoaged skin. British Journal of Dermatology, 151(2), 267–278 (and related Rawlings AV, Harding CR lactic acid publications).*

Studies from Rawlings and colleagues established that regular topical application of lactic acid:
Increases ceramide levels in the stratum corneum — the primary lipid component of the barrier
Reduces transepidermal water loss (TEWL) — improves barrier integrity
Improves the appearance of photoaging (fine lines, roughness, dullness) at 5–12% concentrations over 4–12 weeks

The ceramide-upregulating effect distinguishes lactic acid from glycolic acid — glycolic acid exfoliates without directly driving ceramide synthesis. For patients with compromised barrier function (eczema history, dry skin, topical corticosteroid-induced thinning), lactic acid's barrier-supporting effect makes it a preferable exfoliant.

Keratinocyte desquamation studies

At 5–12%, lactic acid applied daily reduces the desquamation index (degree of abnormal keratinocyte retention) significantly vs. vehicle in dry, scaling skin — evidence for its keratolytic efficacy in the clinically relevant dry-skin population where glycolic acid may be too irritating.

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Concentrations and formulations

Leave-on consumer products (5–12%)

5–8%: Gentle; appropriate for beginners, sensitive skin, and pregnant patients. Suitable for daily use. Some stinging is normal initially; if burning persists, dilute with a moisturizer or reduce frequency.

10–12%: Standard effective concentration for established exfoliant users. More significant keratolytic and brightening effect. Apply every other night to nightly depending on tolerance.

Common leave-on formats: Toners, serums, and moisturizers. Toners and serums at low pH provide the most effective exfoliant concentration. Some moisturizers contain lactic acid at 5–8% as a combined hydrating + exfoliating formulation.

Higher-concentration treatments (15–30%)

15–20%: At-home weekly exfoliant treatments and professional-adjacent products. Significant keratolytic effect; limit contact time (5–10 minutes) if used as a mask/leave-on with higher concentrations.

30%: Professional peel territory. Less common than glycolic acid peels at this concentration; used for dry/sensitive skin types for whom glycolic acid peels are too irritating. Well-tolerated even by Fitzpatrick types IV–VI.

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Lactic acid vs. glycolic acid: when to choose each

| Profile | Preferred AHA | Reason |
|---|---|---|
| Dry or dehydrated skin | Lactic acid | Humectant effect; gentler; ceramide upregulation |
| Oily / acne-prone (non-BHA user) | Glycolic acid | Deeper penetration; stronger exfoliation |
| Rosacea / reactive skin | Lactic acid (5–8%) | Slower penetration; lower irritation |
| Post-procedure recovery | Lactic acid | Less disruptive to recovering barrier |
| Anti-aging primary goal | Glycolic acid | Greater collagen-stimulating evidence (Ditre 1996) |
| Pregnancy | Lactic acid (low %) | Preferred over glycolic; see below |
| Fitzpatrick IV–VI + PIH risk | Lactic or mandelic | Both gentler than glycolic; lower inflammation |

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Safety profile

Photosensitivity

All AHAs increase UV sensitivity by reducing the UV-attenuating capacity of the stratum corneum. Daily SPF 30+ is mandatory when using lactic acid regularly — the same requirement as glycolic acid. Use lactic acid in the evening; apply SPF every morning.

Pregnancy

Lactic acid is generally considered one of the safest exfoliants for use during pregnancy:
Topical absorption is minimal at leave-on concentrations; lactic acid is an endogenous metabolite in humans
Most dermatologists consider 5–10% leave-on lactic acid acceptable during pregnancy
Preferred over salicylic acid (BHA) at significant concentrations, and over glycolic acid, for pregnant patients who require exfoliant use

As with all topical actives during pregnancy, discuss with a physician. Avoid high-concentration peels (> 15%) during pregnancy.

Irritation

Lower than glycolic acid at equivalent concentrations. Signs of over-use: persistent burning (beyond initial application), peeling, erythema lasting more than a few hours. If these occur: reduce frequency, dilute with moisturizer (buffering), or move to a lower concentration.

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How to incorporate lactic acid

Beginners: Start with 5% at pH 3.5–4.0, every other night. Allow 60 seconds absorption before moisturizer. If tolerated for two weeks, advance to nightly.

Intermediate users: 10–12% at pH 3.0–3.5, nightly or alternating with retinoids (lactic acid AM or alternate nights, retinoid on other nights — do not layer both on the same night).

With niacinamide: Compatible. Apply niacinamide after lactic acid (5-minute gap) — niacinamide provides barrier support that complements the exfoliating effect.

Do not layer with: Glycolic acid (stacking AHAs increases irritation without proportional benefit); vitamin C serum on the same application (both at low pH → potential irritation).

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