Formulating Glycolic Acid: Balancing Penetration, Irritation, and Therapeutic Outcomes in Dermatology

Glycolic acid is the most widely used alpha-hydroxy acid (AHA), largely because of its small molecular size (76 Da), which allows more efficient skin penetration than larger AHAs such as lactic acid (90 Da) or mandelic acid (152 Da). Its efficacy depends on concentration and formulation pH relative to its pKa (~3.8), which determines the amount of free acid available for skin interaction. At pH 3.8, approximately 50% of glycolic acid is present as free acid; at pH 4.5, this fraction falls to approximately 18%. In an 8% formulation, about 4% is free acid at pH 3.8, compared with about 1.4% at pH 4.5. This review examines how formulation design influences the penetration, irritation, and therapeutic outcomes of glycolic acid in professional peels and daily use products. Low-pH, high-concentration professional peels (20–70%, pH approximately 1.0–2.5) promote rapid penetration, calcium chelation-mediated corneodesmosome disruption, and controlled exfoliation, but they also increase irritation and transient transepidermal water loss (TEWL). In contrast, buffered daily use formulations (5–15%, pH 3.5–4.5) provide gradual exfoliation, improved tolerability, and longer-term benefits in skin texture, pigmentation, collagen, and glycosaminoglycan synthesis. Overall, the safety and efficacy of glycolic acid are highly formulation-dependent, so different design strategies are needed for daily products and professional peels.