AHA guide: glycolic acid, lactic acid, and how to use chemical exfoliants
A complete guide to alpha hydroxy acids (AHAs) — how glycolic acid, lactic acid, and mandelic acid work, which concentrations are safe for home use, and how to introduce them without irritation.
· By MedSpot Editorial · 6 min read
Alpha hydroxy acids (AHAs) are the most versatile exfoliating category in skincare — with decades of clinical evidence for improving texture, hyperpigmentation, fine lines, and skin clarity. Here's a clear guide to using them correctly.
How AHAs work
AHAs are water-soluble acids derived from natural sources (fruit, milk, almonds) that exfoliate skin by disrupting the bonds between dead skin cells (corneocytes) in the stratum corneum.
The mechanism: At low pH, AHAs cleave the ionic bonds (primarily calcium bridges) that hold desmosomes together — the structures that anchor corneocytes to one another. This releases dead cells from the surface, accelerating the natural desquamation process that would otherwise take 28+ days.
Result: Smoother surface texture, improved luminosity, reduced clogged pores, and — with consistent use — improved collagen synthesis in the dermis (AHAs have secondary stimulation effects below the epidermis at higher concentrations).
AHA types: the key differences
Glycolic acid — the benchmark
Source: Sugarcane
Molecular weight: 76 Da (smallest AHA)
Glycolic acid's small molecular size means it penetrates most rapidly and deeply into the epidermis — which is why it's the most effective and most studied AHA, and also the most irritating.
Best for: Sun damage, fine lines, thickened or rough skin, hyperpigmentation in lighter skin tones, general anti-aging. The workhorse of the AHA category with the most clinical data.
Start concentration: 5–8% for beginners. Professional peels use 20–70%.
Lactic acid — the gentler alternative
Source: Fermented milk (now synthesized)
Molecular weight: 90 Da (slightly larger than glycolic)
Lactic acid is the most recommended AHA for sensitive skin and skin of color. Larger than glycolic, it penetrates more slowly and produces less irritation at equivalent concentrations. It also functions as a humectant — unlike glycolic, it attracts water as it exfoliates, making it inherently more hydrating.
Best for: Dry or sensitive skin, dehydration-related dullness, mild hyperpigmentation, keratosis pilaris. Also the preferred AHA for darker skin tones where glycolic acid carries higher irritation and PIH risk.
Start concentration: 5–10%. Effective in-office peels use 30–70%.
Mandelic acid — the slowest and most tolerated
Source: Bitter almonds
Molecular weight: 152 Da (largest common AHA)
Mandelic acid's large size means the slowest penetration and lowest irritation potential of the AHA group. It's also lipophilic (partially oil-soluble) — an unusual property for an AHA — which helps it penetrate follicles.
Best for: Acne-prone skin with hyperpigmentation, darker skin tones (Fitzpatrick IV–VI) where glycolic risk is higher, rosacea-adjacent skin that needs gentle exfoliation.
Start concentration: 5–10%.
Malic acid and tartaric acid
Less commonly used as standalone actives; often appear as supporting AHAs in multi-acid formulas. Lower potency than glycolic or lactic at equivalent concentrations.
AHA concentration guide
| Concentration | Use | Notes |
|---|---|---|
| 1–5% | Daily leave-on; toners, cleansers | Mild maintenance; suitable for very sensitive skin |
| 5–10% | Effective leave-on treatment; intro range | Most OTC serums; 2–3x/week to start |
| 10–15% | Stronger leave-on; experienced users only | Can irritate; patch test first |
| 20–30% | At-home peels (rinse-off, timed) | Use with timer; do not leave on |
| 30–70% | Professional in-office peels only | Risk of burns; trained providers only |
pH matters as much as concentration
AHAs require a low pH to be active. At pH 3.5–4, AHA molecules are in their undissociated (proton-donor) form and can penetrate stratum corneum. At pH above 4.5, an increasing proportion dissociates into the conjugate base form — which cannot penetrate effectively.
What this means: A "10% glycolic acid" product formulated at pH 6 is largely inactive. A "5% glycolic acid" at pH 3.5 may be more effective. Always check pH when evaluating an AHA product — it matters as much as percentage.
Detection: pH strips can test AHA products; look for formulas that specify pH 3–4 on the label or brand website.
How to introduce AHAs
Introducing AHAs too aggressively causes barrier disruption, sensitivity, and paradoxically worse skin. The right protocol:
Week 1–2: Apply 2× per week. Low concentration (5–8%). Rinse-off or wait 20 minutes before next step (allow skin to buffer back toward neutral).
Week 3–4: Increase to 3× per week if no irritation.
Week 5+: Daily use if tolerated (for lower concentrations). Some people do well with daily use; others stabilize at 3–4× per week.
Signs of overuse: Persistent redness, skin feeling tight or "raw," increased sensitivity, stinging that persists beyond 60 seconds of application. Back off frequency if these appear.
Sun sensitivity and SPF
AHAs increase UV sensitivity — both by removing the thickened surface layer and by increasing cell turnover, which brings newer, less UV-adapted cells to the surface.
Required: Broad-spectrum SPF 30+ every morning when using AHAs. This is non-negotiable — without SPF, AHA use can worsen the pigmentation issues you're trying to treat.
Best practice: Use AHAs at night. Apply SPF in the morning. The increased UV sensitivity is real but manageable with adequate sun protection.
What AHAs treat (by concern)
Sun damage and hyperpigmentation
Glycolic and lactic acid improve hyperpigmentation through accelerated turnover of pigmented cells and some inhibition of melanin transfer. For darker spots — add a dedicated depigmenting agent (vitamin C, niacinamide, azelaic acid) alongside the AHA.
Fine lines and skin texture
Consistent AHA use (12+ weeks) improves fine line appearance and skin roughness through both surface exfoliation and secondary dermal collagen stimulation (documented at higher concentrations).
Dull skin
The most immediate AHA effect — removing the accumulated dead cell layer brightens the complexion within the first 2–4 weeks.
Keratosis pilaris (KP)
Lactic acid 12% is the most evidence-supported topical treatment for KP. The combination of exfoliation and humectancy addresses both the plugged follicle and the surrounding dryness.
Mild acne (blackheads, surface congestion)
AHAs improve surface congestion and comedone formation. For deeper acne, combine with BHA (salicylic acid) for follicular penetration — see our BHA guide for details.
What AHAs don't treat
- Cystic or nodular acne: AHAs work on the surface; cystic acne originates deep in the follicle — BHA and prescription treatments are more appropriate
- Atrophic acne scars: Surface exfoliation doesn't significantly improve structural scarring — professional procedures (microneedling, laser) are indicated
- Deep wrinkles: Fine surface lines improve; structural wrinkles from volume loss do not respond to exfoliation
- Post-inflammatory hyperpigmentation in dark skin with active irritation: Glycolic acid can paradoxically worsen PIH if it irritates the skin — lactic or mandelic are preferred
Layering AHAs
Don't layer AHAs with:
- Retinoids on the same night (early in introduction) — two potent actives amplify irritation. Once skin is adapted, alternating nights or strategic application is possible.
- Vitamin C (L-ascorbic acid) at the same application step — both are low-pH; redundant and irritating. Use vitamin C AM, AHA PM.
- Benzoyl peroxide at the same step — oxidizes some AHA and increases irritation.
Do layer AHAs with:
- Niacinamide (separate step, after AHA) — the barrier support helps counteract AHA-induced TEWL
- Hyaluronic acid or moisturizer after — essential to replenish hydration after exfoliation
- SPF in the AM following
AHA vs. physical exfoliation
Physical exfoliants (scrubs, brushes, dermaplaning) remove dead cells mechanically; AHAs dissolve the bonds between them chemically. AHAs:
- Are more uniform in their effect — no uneven scrubbing pressure
- Can reach into follicles better than surface scrubbing
- Are gentler for sensitive and acne-prone skin (scrubbing can spread bacteria)
- Take longer to show results in some patients but produce longer-lasting improvement
Most dermatologists prefer AHAs over scrubs for the face; physical exfoliation is generally relegated to the body.
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