Lactic Acid

Lactic acid is an alpha hydroxy acid, originally isolated from soured milk. It occupies a useful middle position in the AHA family: more effective than mandelic acid, gentler than glycolic.

Size determines behaviour

Alpha hydroxy acids differ mainly in molecular size, and size determines how fast and how far they penetrate. Glycolic acid is the smallest and acts fastest. Mandelic is the largest and slowest. Lactic acid sits between them.

Faster is not better, it is simply more of everything, including irritation. Lactic acid's position is why it is frequently recommended for skin that finds glycolic acid too much.

What makes it different from the others

Lactate is a component of the skin's own natural moisturising factor, the mix of water-binding compounds held within corneocytes. So lactic acid is a humectant as well as an exfoliant, drawing and holding water while it works.

No other AHA does both to the same degree, which is why lactic acid is described as the hydrating acid and why it suits dry skin better than glycolic does.

How it exfoliates

It interferes with the bonds holding corneocytes together in the outermost layer, allowing them to shed more evenly rather than accumulating unevenly. Longer-term studies have also reported effects on ceramide production and barrier function.

Two forms on ingredient lists

Lactic acid appears in two forms, and the difference is a deliberate formulation choice about how aggressive the product is meant to be.

Lactic acid is the free acid. It sits at a low pH, which makes it more effective and more irritating.

Ammonium lactate is the salt, formed by partially neutralising lactic acid with ammonium hydroxide. Buffering raises the pH toward skin-neutral while leaving a portion of the acid in its free, active form. The result is gentler at the same total concentration, correspondingly less aggressive, and easier to formulate into products that need to sit near neutral pH.

Where you see one or the other on a label, that tells you something about the product's intended strength.

pH, concentration and format

All three matter and only one is usually disclosed. Lactic acid must be partly un-ionised to be active, which depends on pH, and pH is almost never on the label.

Format matters as much. In a rinse-off scrub or cleanser, contact time is under a minute. In a leave-on serum, hours. Comparing percentages across formats is not meaningful.

Sun sensitivity

Alpha hydroxy acids thin the outermost layer as they work, reducing one of the skin's own defences against UV. That effect persists after you stop. Daily broad-spectrum SPF is a requirement of using them rather than an optional extra.