Lactic Acid

Lactic acid is an alpha hydroxy acid, part of the same family as glycolic acid, and the one most often recommended to people who have found glycolic too harsh. It occurs naturally in soured milk, which is where its name and its long folk history come from, though the version used in skincare is produced by fermentation or synthesis rather than extracted from dairy.

The important difference between the two common AHAs is molecular size. Lactic acid is a noticeably larger molecule than glycolic acid, so it does not pass through the surface of the skin as readily. That makes it slower and milder, working more at the surface and less deeply, which is exactly what many people want.

It has a second property that glycolic acid has only weakly: lactic acid is a genuine humectant and part of the skin’s own natural moisturising factor, the mixture of water-binding compounds that keeps the outer layer supple. It therefore draws water into the skin while it exfoliates, which is why it rarely leaves skin feeling stripped in the way stronger acids can.

On an ingredients list it appears as Lactic Acid, sometimes alongside Sodium Lactate, its salt form, which is used purely as a hydrating ingredient rather than an exfoliant. Over-the-counter products usually sit between 5% and 10%, with higher-strength masks and clinic peels going further.

You will find it in toners, serums, overnight masks, cleansers and body lotions, and it is a common choice for dry, dull or mature-looking skin, and for anyone exfoliating for the first time.

Like all alpha hydroxy acids, lactic acid works by weakening the bonds that hold dead cells to the surface of the skin. Those cells normally shed on their own, but the process slows with age, sun exposure and dry conditions, leaving a build-up that makes skin look dull and feel rough, and causes makeup to sit unevenly. Loosening those bonds allows the surface to renew more evenly, so skin looks brighter and feels smoother.

Its larger molecular size means it does this closer to the surface than glycolic acid does. That is a limitation and an advantage at the same time: less reaches the deeper layers, so the effect on the look of fine lines is more modest, but there is correspondingly less stinging, less redness and a gentler experience overall. For dry or easily reddened skin, that trade is usually worth making.

The hydrating side is genuine rather than a formulation trick. Lactic acid binds water within the outer layer and, with consistent use, appears to support the skin’s own production of the compounds that keep that layer supple. This is the reason skin often looks plumper as well as smoother after a few weeks, and why lactic acid is the AHA of choice when dryness is part of the picture rather than something to be worked around.

It still increases sensitivity to daylight, as all AHAs do, so daily sunscreen remains part of the arrangement.

Lactic acid is the sensible starting point for anyone new to exfoliating acids, and often the sensible endpoint too.

• Strength: 5% is a comfortable place to begin. 8–10% suits skin that is already used to acids. Higher-strength masks should be occasional rather than routine.
• Frequency: once or twice a week at first. Many people settle at two or three nights a week, which is plenty.
• When: evening, on clean, dry skin. Wait a minute before the next step.
• Order in the routine: after cleansing, before serums and moisturiser.
• Pairs well with: hyaluronic acid, niacinamide, panthenol, ceramides and squalane. Following it with a good moisturiser makes a noticeable difference.
• Keep apart from: retinoids, vitamin C, benzoyl peroxide, other acids and scrubs on the same evening. Alternate nights instead.
• SPF 30 or above every morning. All AHAs increase sensitivity to daylight.
• Choose it over glycolic acid if your skin is dry, thin, easily reddened or new to acids. Choose glycolic if lactic acid has stopped producing results after a few months.
• On the body, lactic acid lotions work well on rough, dry-looking skin on the arms and legs, where higher strengths are better tolerated.
• Stop if skin feels tight or stings with water. That is over-exfoliation, and the answer is a break and a plain moisturiser.