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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.
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).
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%.
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%.
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%.
Less commonly used as standalone actives; often appear as supporting AHAs in multi-acid formulas. Lower potency than glycolic or lactic at equivalent concentrations.
| 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 |
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.
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.
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.
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.
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).
The most immediate AHA effect — removing the accumulated dead cell layer brightens the complexion within the first 2–4 weeks.
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.
AHAs improve surface congestion and comedone formation. For deeper acne, combine with BHA (salicylic acid) for follicular penetration — see our BHA guide for details.
Don't layer AHAs with:
Do layer AHAs with:
Physical exfoliants (scrubs, brushes, dermaplaning) remove dead cells mechanically; AHAs dissolve the bonds between them chemically. AHAs:
Most dermatologists prefer AHAs over scrubs for the face; physical exfoliation is generally relegated to the body.
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