Glycolic acid vs lactic acid — what they each do, and how to choose.

Glycolic acid and lactic acid are both alpha-hydroxy acids, and both exfoliate by accelerating the shedding of dead surface cells. They are often discussed interchangeably. They are not interchangeable.

The difference between them is primarily molecular weight — and molecular weight determines how deeply each acid penetrates, how aggressively it works, and which skin types are likely to tolerate it without disruption.

What alpha-hydroxy acids do

AHAs are water-soluble exfoliating acids derived from natural sources. They work by loosening the bonds that hold dead corneocytes to the skin's surface — the same cellular bonds that, when left intact, create dull texture, uneven tone, and blocked pores.

The exfoliating effect is pH-dependent: AHAs work only in their acidic, protonated form. Products with a pH above approximately 4.5 produce limited exfoliating activity, regardless of the percentage listed on the label. This is why two products with "10% glycolic acid" can produce very different results — formulation pH matters as much as concentration.

Glycolic acid

Glycolic acid has the smallest molecular weight of any AHA — 76 daltons. Its small size gives it a comparatively deep penetration into the skin's layers. It is the most extensively studied of the AHAs, with clinical evidence behind it for improving texture, reducing fine lines, evening pigmentation, and stimulating collagen synthesis.

This same penetration depth is also what makes glycolic acid the most likely to cause irritation. On skin that is already sensitised, has a compromised barrier, or is being used alongside retinol, glycolic acid can overstimulate the skin's repair response. It is the more aggressive of the two acids.

Most suited to: resilient skin tolerating actives well; people targeting uneven skin tone, textural roughness, or mild fine lines; those whose skin has adapted to acids over time.

Lactic acid

Lactic acid has a larger molecular weight — approximately 90 daltons — and penetrates less deeply than glycolic acid. It exfoliates the surface more gently and is less likely to cause irritation.

It also functions as a humectant: lactic acid draws water to the skin's surface, which glycolic acid does not. This dual action — light exfoliation combined with hydration — makes lactic acid well-suited to skin that is simultaneously trying to address texture and dryness.

The humectant property is also the reason lactic acid is preferred for sensitive, dry, or reactive skin: the exfoliating effect is more superficial, and the concurrent hydration partially buffers the disruption.

Most suited to: sensitive, dry, or reactive skin; beginners introducing acids for the first time; skin that found glycolic acid irritating.

How to choose between them

The decision comes down to two variables: skin resilience and the concern you are targeting.

If the skin has never used exfoliating acids, start with lactic acid. Its gentler penetration profile and humectant action make it less likely to produce a disruptive reaction during the introduction period.

If the skin has adapted to acids, tolerates them without irritation, and the concern is persistent pigmentation or significant textural roughness, glycolic acid's deeper action offers a more pronounced effect.

The concentration matters too. A 5% lactic acid is a gentler introduction than a 10% glycolic acid, even if both products are described as "exfoliating acids." Start with a lower concentration of either and increase only if the skin's response is stable.

What both have in common

Both glycolic and lactic acid increase the skin's sensitivity to UV radiation — a well-documented AHA effect. When using either acid, daily broad-spectrum SPF 30 or higher is not optional. The exfoliation removes the outer layer of cells; the skin beneath is fresher, more photosensitive, and more vulnerable to photodamage. Using an acid without sunscreen is counterproductive.

Both should be applied in the evening. Both should be used two to three times per week at most, rather than daily, and not on the same evenings as retinol.

The Lux & Glo position

The ritual does not include an exfoliating acid. The oil cleanser and niacinamide serum are designed to support barrier function and manage sebum and tone without active exfoliation. For people who want to add an acid, lactic acid at a modest concentration — two evenings per week, not on retinol nights — is the lower-disruption starting point. Glycolic acid is a reasonable step up once the skin has adapted.

Both are effective. The question is which one the skin is ready for.