The smallest, most penetrating AHA. At 5-10% it smooths texture and evens tone over months, but raises sunburn risk while you use it.
Glycolic acid is the smallest alpha-hydroxy acid (AHA). It loosens the bonds between dead cells in the outer layer so they shed faster, which smooths and brightens skin; with long use at higher strengths it also thickens the epidermis and increases dermal collagen.
Strength depends on both percentage and pH: the lower the pH, the more free acid and the stronger (and more irritating) the effect. The US cosmetic ingredient safety panel considers AHAs safe in consumer products up to 10% at pH 3.5 or higher; 20-70% peels belong in a clinic.
A 22-week double-blind trial in 74 women found 8% glycolic acid cream improved overall photodamage more than vehicle (76% vs 40% improved by at least one grade), and a 25% lotion thickened skin and raised collagen density in a biopsy study. Effects at consumer strengths are modest. A controlled study found 10% glycolic acid for 4 weeks made skin burn more easily in UV light.
74 women with photodamaged skin used 8% glycolic acid, 8% L-lactic acid or vehicle cream twice daily for 22 weeks; at least one grade of facial improvement was seen in 76%, 71% and 40% respectively. Lactic acid also improved mottled pigment and roughness on the forearms. Benefits were modest and the creams well tolerated.
Lotions with 25% glycolic, lactic or citric acid applied to one forearm versus placebo on the other for about 6 months increased skin thickness by about 25%, with denser collagen and better elastic fibres on biopsy.
In 29 adults, 4 weeks of 10% glycolic acid (pH 3.5) increased UV-induced sunburn cells and lowered the dose of UV needed to cause redness versus placebo; the effect was gone a week after stopping.
Reviews alpha-hydroxy, beta-hydroxy and polyhydroxy acids, their uses for photoaging, acne and pigmentation, and their effects on sun sensitivity, noting that their mechanisms of action still need clarification.