Lactic Acid

A three-carbon alpha-hydroxy acid (90 Da) and a native component of the skin's natural moisturising factor — meaning topical application both exfoliates and supplements the endogenous NMF pool. Disrupts corneocyte cohesion at low pH but the larger molecular size (relative to glycolic) slows penetration and reduces irritation. Distinct from other AHAs in two ways: stimulates ceramide synthesis in keratinocytes (the Rawlings 1996 finding that anchors its barrier-positive reputation) and humectant-binds water through its carboxylate group. L-lactic acid (the biologically native enantiomer) is the cosmetic-active form; racemic DL is functionally equivalent on exfoliation but lacks the NMF identity.

5–10% leave-on (pH 3.5–4); 5% is the Sunday Riley Good Genes hero dose

5–10% leave-on at pH 3.5–4 is the sweet spot. L-lactic acid (the bio-derived form) is preferred over racemic DL-lactic. Pairs naturally with hyaluronic acid and ceramides for hydration-led resurfacing.

Photosensitizing — daily SPF required. Over 10% requires acid tolerance.

Photosensitisation is real but lower than glycolic in head-to-head comparison. The 'natural lactic acid from fermented milk' marketing on some clean lines obscures that cosmetic-grade L-lactic is overwhelmingly from corn-glucose fermentation. EU sub-4% restriction discussion has touched lactic peripherally but no current cap.

Sold as the gentle AHA, and uniquely earns the positioning because of the ceramide and NMF mechanisms — it's the only AHA that strengthens the barrier as a side effect of exfoliating it. Sunday Riley Good Genes is the canonical product; the molecule is one of the most-recommended in inclusive dermatology.