Skin Actives

Alpha Hydroxy Acids (AHAs)

Also known as: AHA, Glycolic acid, Lactic acid, Mandelic acid

Water-soluble surface exfoliants — glycolic, lactic, and friends — that smooth texture, brighten, and boost hydration.

By IngredientWiki Editorial Team
Fact-checked by IngredientWiki Editorial Team
Updated 2026-09-25
3 min read

In short

Alpha hydroxy acids (AHAs) are a family of water-soluble acids used in skincare to exfoliate the skin's surface by loosening the bonds between dead cells. Glycolic, lactic, citric, malic, and mandelic acids differ mainly in molecule size and gentleness. Regular use smooths rough texture, brightens dullness, and fades dark spots, with lactic acid adding hydration and glycolic acid offering the most dramatic resurfacing.

Water-soluble acids that dissolve dead-cell bonds on the surface
Most common: glycolic (strongest) and lactic (gentler, hydrating)
Daily products: 5–10%; professional peels: 20–70%
Increase sun sensitivity — daily sunscreen is essential

What is Alpha Hydroxy Acids (AHAs)?

AHAs are the skincare industry's original chemical exfoliants, tracing back to Cleopatra's sour-milk baths — milk contains lactic acid. All AHAs work the same basic way: they weaken the calcium-dependent bonds holding dead skin cells together, so the dull, spent outer layer sheds evenly instead of clinging in flakes. Different AHAs trade off strength and gentleness based on molecular size: smaller molecules penetrate faster (glycolic), larger ones work more superficially (mandelic, lactic).

Where it comes from

Natural sources

Glycolic acid: sugarcane
Lactic acid: milk, fermented foods (now usually synthetic)
Citric acid: citrus fruits
Malic acid: apples; mandelic acid: bitter almonds

How it's made

Most cosmetic AHAs are produced synthetically for purity and consistency.

What it does in products

Benefits

Smoother texture and glow

strong evidence

By shedding the rough outer layer, AHAs restore light reflection and smoothness — often visible within days, with best results after 4–8 weeks.

Fading dark spots and melasma

strong evidence

Glycolic acid is a standard treatment for hyperpigmentation; trials show significant fading of sun spots and melasma, especially combined with other brighteners.

Fine-line softening and collagen support

moderate evidence

Long-term AHA use (notably glycolic) has been shown to increase dermal thickness and improve signs of photoaging — among the few OTC actives with this kind of evidence.

Hydration (lactic acid)

moderate evidence

Lactic acid doubles as a humectant, making it the preferred AHA for dry and sensitive skin.

Side effects & safety

Stinging, redness, and peeling at first (common)Expected during acclimation; start at 5% or less, two to three times per week.

Increased sun sensitivity (common)AHAs thin the protective dead layer; broad-spectrum SPF 30+ daily is non-negotiable while using AHAs.

Chemical burn from high-strength peels (rare)20%+ peels belong in professional hands or require strict protocol adherence.

Safety considerations

Low-concentration glycolic and lactic acid products are widely considered acceptable in pregnancy; most guidance cautions against high-strength peels. Check with your provider.
Do not layer with retinoids or BHAs in the same routine step — space them across days.
AHAs lower the skin's pH, which can destabilize certain other actives; apply them first, let them absorb, then continue.
Not for: Active sunburn, open wounds, very sensitive or rosacea-prone skin (choose lactic or mandelic instead)

How it's used

In skincare

Daily products: 5–10% (glycolic often 7–8%, lactic 5–10%). Start two to three nights per week. Professional peels run 20–70% and are performed in-office. Always pair with sunscreen.

Regulatory status

Cosmetic ingredients under the FD&C Act at everyday concentrations; professional-strength peels are used under medical supervision.
INCI name on cosmetic labels: Glycolic Acid / Lactic Acid / Citric Acid / Malic Acid / Mandelic Acid.
The CIR Expert Panel has assessed AHAs as safe in cosmetics at low concentrations with pH and sunscreen labeling guidance.

Choosing your AHA

Glycolic acid, the smallest molecule, penetrates deepest — maximum resurfacing, maximum potential irritation. Lactic acid is larger and hydrating — the best pick for dry or sensitive skin. Mandelic acid is the largest and gentlest — favored by darker skin tones because its slow penetration is associated with less risk of post-inflammatory hyperpigmentation. Citric and malic acids mostly play supporting roles in blends.

AHAs compared
Acid	Molecule size	Strength	Best for
Glycolic	Smallest	Strongest	Texture, dark spots, oiliness
Lactic	Medium	Moderate	Dry or sensitive skin, hydration
Mandelic	Largest	Gentlest	Sensitive and deeper skin tones
Citric / Malic	Medium	Mild	Supporting roles in blends

Frequently asked questions

How often should I use AHAs?

Begin two to three nights per week and increase to every other night as tolerated. If your skin stings, flakes, or looks glassy, pause and rebuild the barrier with moisturizer. More frequent use is not better — steady use is.

Can I use AHAs in the morning?

You can, but they increase sun sensitivity, so night use is safer and maximizes results while you sleep. If used in the morning, strict daily sunscreen is mandatory.

Are AHAs safe for dark skin?

Yes, with care. Gentle options (lactic, mandelic) and slow introduction minimize the risk of irritation-induced dark spots. Avoid aggressive peeling and burning — irritation itself can trigger hyperpigmentation in deeper skin tones.

The bottom line

AHAs are the most reliable OTC tools for dullness, rough texture, and dark spots. Choose the acid that fits your skin — glycolic for toughness and texture, lactic for dryness, mandelic for sensitivity — start low and slow, and treat sunscreen as part of the product, not an afterthought.

References

Glycolic acid compound summary. PubChem (National Library of Medicine). https://pubchem.ncbi.nlm.nih.gov/compound/757
Alpha hydroxy acids in dermatology — literature search. PubMed (National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/?term=alpha+hydroxy+acid+skin+aging
Effects of alpha-hydroxy acids on photoaged skin. Journal of the American Academy of Dermatology. https://pubmed.ncbi.nlm.nih.gov/8642081/
Cosmetic Ingredient Review — AHA safety assessment. Cosmetic Ingredient Review. https://www.cir-safety.org/ingredients