Quick Comparison
| Lactic Acid | Vitamin C (L-Ascorbic Acid) | |
|---|---|---|
| Typical Concentration | Concentrations: 5-12% for daily use. 30-50% for professional peels. Start with 5% every other night. The Ordinary offers 5% (gentle) and 10% (moderate) options. Always use SPF during the day. | L-Ascorbic Acid: 10-20% at pH 2.5-3.5. Start with 10% if new to vitamin C. Apply in the morning under sunscreen for photoprotective synergy. The SkinCeuticals CE Ferulic formula (15% L-AA + 1% vitamin E + 0.5% ferulic acid) is the most studied and copied formulation. |
| Application | Topical (serum, peel, toner). Apply to dry skin at night. Follow with moisturizer. | Topical (serum, usually water-based). Apply to clean, dry skin in the morning before sunscreen. Store in cool, dark place. Discard when it turns dark yellow or brown. |
| Research Papers | 10 papers | 10 papers |
| Categories |
Mechanism of Action
Lactic Acid
Lactic acid (90 Da, larger than glycolic acid) exfoliates via the standard AHA mechanism: chelating calcium at corneodesmosomes and promoting desquamation through protease activation. Unlike glycolic acid, lactic acid is a natural component of the skin's natural moisturizing factor (NMF) and functions as a humectant, drawing water into the stratum corneum through hygroscopic binding. It inhibits tyrosinase enzyme activity in melanocytes, providing mild brightening. At higher concentrations (10%+), lactic acid upregulates serine palmitoyltransferase and glucosylceramide synthase in keratinocytes, stimulating ceramide synthesis and improving barrier lipid composition. It also enhances filaggrin proteolysis to NMF components. This dual action—exfoliation plus barrier support—makes it the most moisturizing AHA and clinically useful for dry, sensitive, or barrier-compromised skin.
Vitamin C (L-Ascorbic Acid)
L-Ascorbic acid donates electrons to scavenge reactive oxygen species (superoxide, hydroxyl radical, singlet oxygen) and reactive nitrogen species from UV, pollution, and metabolism—preventing oxidative damage to lipids, proteins, and DNA. It inhibits tyrosinase (copper enzyme catalyzing tyrosine to L-DOPA to dopaquinone) through copper chelation and competitive inhibition. Ascorbate is an essential cofactor for prolyl and lysyl hydroxylase—enzymes that hydroxylate collagen residues for triple-helix formation and lysyl oxidase crosslinking. Vitamin C regenerates oxidized vitamin E, creating a sustained redox cycle. Ferulic acid stabilizes both vitamins; the CE Ferulic combination provides 4-8x greater photoprotection than vitamin C alone. Penetration requires pH 2.5-3.5.
Risks & Safety
Lactic Acid
Common
Mild stinging, redness — less than glycolic acid at equivalent concentrations. Sun sensitivity.
Serious
None at cosmetic concentrations.
Rare
Over-exfoliation with daily high-concentration use.
Vitamin C (L-Ascorbic Acid)
Common
Tingling/stinging on application (due to low pH), oxidation of product (turns yellow/brown — discard when this happens).
Serious
None.
Rare
Contact dermatitis, especially with oxidized product. May cause temporary orange staining of skin at high concentrations.
Full Profiles
Lactic Acid →
A gentle AHA derived from milk that provides chemical exfoliation plus hydration — a unique dual benefit. Lactic acid has a larger molecular size than glycolic acid, so it penetrates more slowly and causes less irritation, making it ideal for sensitive skin and AHA beginners. It also has humectant properties, drawing moisture into the skin as it exfoliates.
Vitamin C (L-Ascorbic Acid) →
The most potent topical antioxidant with strong clinical evidence for brightening, anti-aging, and photoprotection. L-Ascorbic Acid is the pure, active form that directly neutralizes free radicals, inhibits melanin production, and stimulates collagen synthesis. The challenge is formulation — it is notoriously unstable and must be at low pH (2.5-3.5) for skin penetration, which can cause irritation.