Quick Comparison
| Hydroquinone | Lactic Acid | |
|---|---|---|
| Typical Concentration | OTC (where available): 2%. Prescription: 4%. Apply to dark spots only (not entire face) once or twice daily. Use in 3-4 month cycles with 2-3 month breaks. Always use with sunscreen (SPF 30+) — without it, hydroquinone is ineffective. | 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. |
| Application | Topical (cream, gel, serum). Apply precisely to hyperpigmented areas. Avoid contact with normal skin. | Topical (serum, peel, toner). Apply to dry skin at night. Follow with moisturizer. |
| Research Papers | 9 papers | 10 papers |
| Categories |
Mechanism of Action
Hydroquinone
Hydroquinone inhibits tyrosinase through multiple mechanisms: competitive alternative substrate, oxidation to semiquinone radicals generating ROS that damage melanocyte mitochondria and ER, copper chelation at tyrosinase active site. Inhibits RNA/DNA synthesis via ribonucleotide reductase interference. Causes melanosome degradation through membrane disruption. Dramatic melanin reduction — eumelanin and pheomelanin pathways suppressed. Selectively affects hyperactive melanocytes, sparing quiescent ones. Fades pigmentation without permanently altering baseline skin color. Pigmentation returns when treatment stops (melanocyte stem cells intact). Enhanced with retinoids (penetration) and sunscreen (prevents UV rebound).
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.
Risks & Safety
Hydroquinone
Common
Mild redness, stinging, dryness.
Serious
Exogenous ochronosis (paradoxical blue-grey darkening) with prolonged use >6 months, particularly in darker skin tones. Irritant and allergic contact dermatitis.
Rare
Nail discoloration, peripheral neuropathy (extremely rare, systemic exposure).
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.
Full Profiles
Hydroquinone →
The most potent topical depigmenting agent available and the gold standard prescription treatment for melasma and stubborn hyperpigmentation. Hydroquinone inhibits tyrosinase more strongly than any OTC alternative. However, it carries risks with long-term use (ochronosis — paradoxical darkening) and is restricted or banned in many countries. Best used in cycles of 3-4 months under dermatologist guidance.
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.