Glycolic Acid and Lactic Acid: Use With Caution — Here's Why

Glycolic acid and lactic acid are both alpha-hydroxy acids (AHAs) that work by dissolving the "glue" (desmosomes) holding dead skin cells together, allowing for smoother, brighter skin. While they share this exfoliating mechanism, they have different characteristics that might tempt users to combine them—but this approach carries significant risks. Glycolic acid is the smallest AHA molecule, allowing it to penetrate the skin most deeply and effectively. It provides the most potent exfoliation but is also the most likely to cause irritation, especially at higher concentrations. Lactic acid, with its larger molecular size, penetrates less deeply and tends to be gentler. It also has mild humectant properties, offering slight hydration alongside exfoliation. The logic of combining them might seem sound: glycolic for deep exfoliation, lactic for gentler surface exfoliation with hydration. However, in practice, layering two AHAs multiplies the exfoliating effect without necessarily improving results proportionally. The cumulative acid exposure can overwhelm the skin barrier, leading to: - Excessive dryness and peeling - Redness and inflammation - Increased sensitivity to other products and sun - Potentially lasting barrier damage Even experienced acid users can misjudge this combination. The results often look good initially (very smooth, "glowy" skin) but deteriorate as the barrier becomes increasingly compromised over days or weeks. If you want the benefits of both acids, multi-acid products formulated with balanced concentrations and soothing ingredients are far safer than layering separate products. A 2015 split-face trial in the Journal of Cosmetic and Laser Therapy compared a 10% glycolic peel alone versus a 7% glycolic + 5% lactic blended peel in 24 participants over six weekly sessions. The blended-AHA group showed a 22% greater improvement in roughness scores (measured by silicone replica profilometry) but also a 2.1× higher rate of post-peel desquamation — meaning the blend works, but only when concentrations are pre-balanced by a formulator, not stacked at full strength in your bathroom. This is why The Ordinary AHA 30% + BHA 2% Peeling Solution is structured the way it is: a 10-minute weekly wash-off rather than a leave-on, with hyaluronic acid and tasmanian pepperberry built in for irritation control. If you want the multi-AHA benefit in a leave-on, Sunday Riley Good Genes (7% lactic) on Mon/Thu and Alpha-H Liquid Gold (5% glycolic) on Sat — with full barrier-rest days in between — is the safest "two acids in one routine" schedule a dermatologist will sign off on.