When dermatologists and skincare enthusiasts discuss alpha-hydroxy acids, two names dominate the conversation: glycolic acid and lactic acid. Both exfoliate. Both improve texture and tone. Both have decades of research supporting their effectiveness. Yet they produce meaningfully different experiences on skin—and the primary reason comes down to something you can't see or feel directly: molecular size.
Understanding this difference helps explain why some people thrive on glycolic acid while others find it intolerable, and why lactic acid has become the preferred AHA for sensitive, reactive, or easily irritated skin.
The Size That Matters
Glycolic acid has a molecular weight of 76.05 g/mol, making it the smallest alpha-hydroxy acid used in skincare. Lactic acid's molecular weight is 90.08 g/mol—approximately 18% larger.
This difference might seem trivial on paper, but it translates to substantial differences in how each acid interacts with skin. Smaller molecules penetrate faster and deeper. They pass through the stratum corneum more easily and reach the living layers of the epidermis more quickly.
For glycolic acid, this rapid penetration produces fast, visible results—but it also means the acid is more likely to trigger inflammation, especially in skin that's already compromised or naturally reactive. The speed of penetration doesn't give skin time to modulate its response.
Lactic acid's larger size means it penetrates more gradually and works more superficially. It still reaches the dead cell layers that need exfoliation, but it's less likely to venture into territory where it causes collateral damage. This is why people who can't tolerate glycolic acid often find lactic acid perfectly manageable.
Beyond Size: The Humectant Advantage
Molecular size isn't the only difference between these acids. Lactic acid has a property that glycolic acid lacks: it's a humectant.
Humectants are substances that attract and bind water. When you apply lactic acid to skin, it draws moisture from the environment and from deeper skin layers, helping to maintain hydration at the surface. This built-in moisturizing effect counteracts the potential drying impact of exfoliation.
Glycolic acid offers no such benefit. While it's not inherently drying, it doesn't contribute to hydration either. This means glycolic acid products often require careful formulation with additional moisturizing ingredients to prevent the tight, stripped feeling that can follow exfoliation.
For people with dry skin—or for anyone using exfoliating acids in dry climates or harsh weather—lactic acid's humectant properties provide a significant practical advantage.
The Irritation Calculus
Any acid applied to skin creates some degree of stress. The question is whether your skin can manage that stress productively or whether it tips into dysfunction.
Glycolic acid's aggressive penetration profile means it's more likely to trigger:
Barrier disruption. The stratum corneum isn't just dead cells waiting to be removed; it's an active protective structure. Glycolic acid can penetrate past the cells that actually need removal and disturb the living layers beneath.
Inflammatory response. When skin perceives damage, it responds with inflammation—redness, heat, swelling. Glycolic acid's deep penetration can trigger this response even when the "damage" is actually intended exfoliation.
Sensitization over time. Some people tolerate glycolic acid initially but develop sensitivity with continued use. The repeated stress gradually depletes the skin's capacity to recover, leading to progressive intolerance.
Lactic acid's gentler profile reduces all three risks. It still exfoliates—the dead cells still shed—but it does so without pushing skin into a defensive state. For many people, this means sustainable, long-term use without the cumulative sensitivity that can develop with more aggressive acids.
The Ceramide Connection
Research has identified another advantage specific to lactic acid: it appears to stimulate ceramide production in the skin.
Ceramides are lipid molecules that form the "mortar" between the "bricks" (corneocytes) of your stratum corneum. They're essential for barrier function—for keeping moisture in and irritants out. Ceramide levels naturally decline with age and can be depleted by harsh skincare products, environmental stress, and certain skin conditions.
Studies suggest that lactic acid signals skin to produce more ceramides, potentially strengthening the barrier even as exfoliation removes surface cells. This creates a genuinely different dynamic than glycolic acid, which exfoliates without contributing to barrier repair.
For people with compromised barriers—whether from eczema, rosacea, overuse of actives, or simply aging—this ceramide-stimulating property makes lactic acid particularly appealing.
When Glycolic Makes Sense
None of this means glycolic acid is bad or that everyone should avoid it. For people with resilient skin that tolerates aggressive treatment, glycolic acid's rapid penetration delivers faster results. Professional chemical peels often use glycolic acid precisely because of its ability to reach deeper layers.
Glycolic acid may also be preferable for:
Acne-prone skin that needs deeper penetration to address congested pores. The aggressive action that irritates sensitive skin can benefit oily, acne-prone skin that's built to handle it.
Professional treatments where controlled, supervised application can leverage glycolic acid's strengths while managing its risks.
Younger skin with robust barrier function and high cell turnover that can easily recover from aggressive exfoliation.
When Lactic Makes More Sense
Lactic acid becomes the better choice for:
Sensitive or reactive skin that flares easily from potent actives. The gentler penetration profile reduces the risk of inflammatory responses.
Dry skin that needs exfoliation without additional moisture loss. Lactic acid's humectant properties work with dry skin rather than against it.
Mature skin that may benefit from ceramide stimulation along with exfoliation. The dual action supports barrier function while improving texture.
Damaged barriers from overuse of actives, environmental stress, or skin conditions. Lactic acid is less likely to exacerbate barrier dysfunction.
Long-term maintenance where sustainable, everyday use matters more than aggressive short-term results.
The Goat Milk Advantage
On our Washington State farm, we formulate with fresh goat milk—which naturally contains lactic acid—rather than adding synthetic lactic acid to our products. This matters because goat milk delivers lactic acid within a complex biological matrix of fats, proteins, and other nutrients that naturally buffer and support the acid's activity.
The lactic acid in goat milk doesn't hit skin as an isolated compound. It arrives surrounded by natural emollients that protect the skin barrier, proteins that may reduce irritation, and additional nutrients that support skin health. This is fundamentally different from applying a synthetic lactic acid serum, even if the lactic acid molecule itself is identical.
Combined with MSM in every Artisan formula—which provides additional anti-inflammatory support—this approach offers the exfoliating benefits of lactic acid with built-in safeguards that isolated acids can't provide.
Making the Right Choice
The glycolic-versus-lactic decision isn't about which acid is objectively superior. It's about matching the acid's properties to your skin's needs and tolerances.
If you've used glycolic acid successfully for years with no issues, there's no compelling reason to switch. But if you've experienced progressive dryness, increased sensitivity, or persistent irritation, lactic acid offers a way to maintain exfoliation benefits while giving your skin a chance to recover.
The best skincare isn't about using the most aggressive ingredients. It's about matching your ingredients to your skin's actual needs—and for many people, that match is lactic acid.
