Glycolic Acid vs Lactic Acid: Which AHA Is Right for You?

Glycolic acid and lactic acid are the two most widely used alpha hydroxy acids (AHAs) in skincare. Both exfoliate the skin surface by disrupting the bonds between dead cells, but they differ in molecular size, penetration depth, potency, and secondary effects. Glycolic acid is smaller and more penetrating; lactic acid is larger, gentler, and doubles as a humectant. Understanding these differences is the key to choosing the right AHA for your skin type and concerns.

The Science: Why Molecular Weight Matters

Alpha hydroxy acids work by breaking the calcium-ion bridges that hold corneodesmosomes together - the protein structures that bind dead skin cells in the stratum corneum [1]. The smaller the acid molecule, the more easily it penetrates through the tightly packed layers of dead cells, and the deeper it reaches before losing its chemical potency.

Glycolic acid has a molecular weight of 76.05 Da, making it the smallest AHA. Lactic acid has a molecular weight of 90.08 Da - only about 18% larger, but that difference is enough to measurably affect penetration depth and the speed of exfoliation [2].

This is not a theoretical distinction. Fartasch et al. (1997) demonstrated through electron microscopy that glycolic acid at just 4% concentration causes targeted breakdown of desmosomes in the stratum disjunctum (the outermost layer of dead cells) while leaving the deeper stratum compactum and its barrier lipid structures intact [3]. Lactic acid produces a similar exfoliating mechanism but with less rapid and less deep penetration at equivalent concentrations, owing to its larger molecular size.

Glycolic Acid: Profile and Strengths

Glycolic acid is derived from sugarcane and is the workhorse of the AHA family. Its small molecular size allows it to penetrate the stratum corneum more uniformly and deeply than any other AHA, which translates to several specific strengths.

Surface Texture and Smoothing

Glycolic acid excels at smoothing rough, uneven skin texture. By efficiently removing the outermost layers of dead cells, it creates a more uniform skin surface that reflects light more evenly - producing the characteristic "glow" that people associate with AHA use. No other commonly available AHA matches glycolic acid's resurfacing ability at over-the-counter concentrations [1].

Collagen Stimulation

Bernstein et al. (2001) demonstrated that topical application of 20% glycolic acid lotion for three months significantly increased type I collagen mRNA expression and hyaluronic acid content in human skin [4]. While this study measured mRNA expression (not necessarily protein deposition), subsequent in vitro work by Kim et al. (1998) confirmed that glycolic acid stimulates greater collagen synthesis than lactic acid in dose-dependent fashion [5].

A 2025 network meta-analysis of 23 RCTs and 3,905 participants confirmed that glycolic acid is effective for reducing skin roughness, though it did not reach statistical significance for fine wrinkle reduction - a distinction where retinoids remain superior [6].

Hyperpigmentation

Glycolic acid addresses hyperpigmentation through two mechanisms: it accelerates cell turnover to disperse melanin deposits in the epidermis [1], and it directly inhibits tyrosinase activity to suppress melanin production [7]. A 2025 RCT comparing 50% glycolic acid peels to 80% lactic acid peels for melasma found glycolic acid more effective, with a median MASI score decrease of 2.85 vs 1.8 for lactic acid (P=0.009) [8].

Acne Treatment

A large RCT (n=120) demonstrated that 10% glycolic acid applied daily significantly improved mild acne compared to placebo, with visible improvement by day 45 [9]. Glycolic acid also has pH-dependent antibacterial activity against Cutibacterium acnes, the primary acne-causing bacterium, at concentrations as low as 0.2% [10].

Lactic Acid: Profile and Strengths

Lactic acid is naturally found in the human body as part of the skin's natural moisturizing factor (NMF). It is derived from milk sugars (hence the name, from the Latin lac for milk) and has a long history in skincare - Cleopatra's legendary milk baths were, in effect, lactic acid treatments.

Gentleness and Tolerability

Lactic acid's slightly larger molecular size means it penetrates more slowly and less deeply than glycolic acid at equivalent concentrations and pH. This makes it inherently less irritating, which is why dermatologists frequently recommend it as a starting AHA for sensitive skin types or AHA beginners [2].

The slower penetration also means lactic acid is less likely to cause the stinging, redness, and peeling that glycolic acid can produce during the initial adjustment period. For people whose skin barrier is easily compromised, this gentler approach can produce meaningful exfoliation without triggering a flare of irritation or side effects.

Humectant Properties

Unlike glycolic acid, lactic acid functions as a humectant - it draws moisture from the environment and the deeper layers of skin into the stratum corneum. This dual action (exfoliation + hydration) makes lactic acid uniquely suited for dry skin types. Where glycolic acid can sometimes leave skin feeling tight or dry during the initial weeks of use, lactic acid is less likely to create that sensation because it simultaneously increases the skin's moisture content [2].

The humectant property also means that lactic acid products tend to have a more cosmetically elegant feel on the skin, and they layer well under moisturizers without the same risk of dryness-related sensitivity.

Efficacy for Photodamage

Stiller et al. (1996) conducted a double-blind, vehicle-controlled RCT comparing 8% glycolic acid cream to 8% lactic acid cream for photodamaged skin over 22 weeks. The study found that both acids were "modestly useful in ameliorating signs of chronic cutaneous photodamage," with 76% of glycolic acid users achieving at least one grade of improvement. Lactic acid produced comparable results, though glycolic acid showed a slight edge in overall effectiveness [11].

This is an important study because it compared the two acids head-to-head at the same concentration in a rigorous trial design. The takeaway: lactic acid works for photodamage, but glycolic acid has a measurable advantage.

Suitability for Darker Skin Tones

Lactic acid's gentler penetration profile gives it a potential advantage for Fitzpatrick skin types IV through VI, where aggressive exfoliation can trigger post-inflammatory hyperpigmentation (PIH). While glycolic acid peels have been used safely in darker skin with appropriate protocols [12], the inherently lower irritation risk of lactic acid may make it a safer first-line choice for people with melanin-rich skin who want to minimize PIH risk.

Head-to-Head Comparison

For Anti-Aging

Winner: Glycolic acid.

The evidence for glycolic acid's collagen-stimulating and texture-improving properties is stronger than for lactic acid. Bernstein et al. (2001) demonstrated molecular-level changes (increased collagen mRNA and hyaluronic acid) with glycolic acid [4], and Kim et al. (1998) found glycolic acid produced greater collagen synthesis than lactic acid [5]. If your primary concern is photoaging, rough texture, or loss of firmness, glycolic acid is the more evidence-backed choice.

Caveat: For fine wrinkle reduction specifically, retinoids remain superior to both AHAs [6]. Learn more about how glycolic acid works at the molecular level.

For Hyperpigmentation

Winner: Glycolic acid.

The 2025 RCT comparing glycolic acid peels to lactic acid peels for melasma found glycolic acid significantly more effective, even when the lactic acid concentration was much higher (50% GA vs 80% LA) [8]. Glycolic acid's combination of faster cell turnover and direct tyrosinase inhibition [7] gives it a meaningful advantage for dark spots, sun damage, and post-inflammatory hyperpigmentation.

For Sensitive Skin

Winner: Lactic acid.

If your skin is reactive, easily irritated, or prone to redness, lactic acid is the safer starting point. Its larger molecular size means slower, shallower penetration and less acute irritation. The humectant effect also helps buffer against the dryness that can exacerbate sensitivity. Start with 5% lactic acid 2-3 times per week and increase gradually.

For more on adapting acid use to your specific skin type, see our skin types guide.

For Dry Skin

Winner: Lactic acid.

Lactic acid's humectant properties make it uniquely suited for dry skin. It exfoliates while simultaneously drawing moisture into the skin, addressing two concerns at once. Glycolic acid can exacerbate dryness in the initial weeks of use, particularly at higher concentrations or lower pH values.

For Texture and Roughness

Winner: Glycolic acid.

This is glycolic acid's core strength. Its smaller molecular size allows more uniform and deeper penetration across the stratum corneum, producing smoother, more evenly exfoliated skin. For stubborn rough patches, keratosis pilaris, or overall dullness, glycolic acid delivers faster and more noticeable results [1].

For Beginners

Winner: Lactic acid.

If you have never used an AHA before and are unsure how your skin will respond, lactic acid is the lower-risk starting point. You can always progress to glycolic acid later once you know your skin tolerates chemical exfoliation well. Starting with glycolic acid is not dangerous, but it is more likely to produce initial irritation that could discourage continued use.

Concentration Ranges Compared

Both acids are available across a wide range of concentrations, but the effective ranges differ because of their different potencies at equivalent percentages.

Lactic acid over-the-counter products typically range from 5% to 12%. At these concentrations, lactic acid provides gentle to moderate exfoliation suitable for regular home use.

Glycolic acid over-the-counter products typically range from 5% to 10%. The CIR Expert Panel has established 10% at pH 3.5 or higher as the upper safety limit for consumer cosmetic products in the US [13]. The EU is more conservative, limiting consumer leave-on products to 4% glycolic acid at pH 3.8 or higher [14].

For professional peels, glycolic acid concentrations range from 20% to 70%, while lactic acid peels typically use 30% to 90% - the higher concentrations needed because lactic acid is less potent per percentage point.

Remember that concentration alone does not determine potency. The pH of the product determines how much of the acid is in its active (non-ionized, or "free acid") form. A 10% glycolic acid product at pH 3.5 has a free acid value of approximately 6.8%, while the same concentration at pH 4.5 has a free acid value of only about 1.8% [15]. Use our free acid calculator to compare the actual potency of products.

Can You Use Both?

Yes. Glycolic acid and lactic acid can be used together or alternated, and some multi-acid formulations combine them deliberately. Because they share the same mechanism of action (desmosome disruption in the stratum corneum), the main risk of combining them is over-exfoliation - not a chemical incompatibility.

Safe Approaches

Alternate products. Use a glycolic acid product on some evenings and a lactic acid product on others. For example, glycolic acid 2-3 nights per week for its deeper exfoliation, and lactic acid on the remaining nights for gentler maintenance with added hydration.

Seasonal rotation. Some people use glycolic acid during cooler months (when UV exposure is lower and the skin can tolerate more aggressive exfoliation) and switch to lactic acid during summer for gentler exfoliation with less photosensitivity risk.

Different areas. Apply glycolic acid to areas that need deeper exfoliation (forehead, chin, areas of stubborn texture) and lactic acid to more sensitive areas (cheeks, around the eyes, neck).

Multi-acid formulations. Many commercial products combine glycolic and lactic acid at lower individual concentrations. These blends aim to provide some of the penetration depth of glycolic acid with the gentleness and hydration of lactic acid.

How to Choose: Decision Framework

Choose glycolic acid if:

• Your primary concern is rough, uneven skin texture
• You want to address signs of photoaging (roughness, dullness, loss of radiance)
• You are dealing with stubborn hyperpigmentation or dark spots
• Your skin is normal to oily and not particularly sensitive
• You have used AHAs before and tolerate them well

Choose lactic acid if:

• You have sensitive or reactive skin
• You are new to chemical exfoliation and want to start gently
• Your skin is dry and you want exfoliation plus hydration
• You have a darker skin tone and want to minimize PIH risk
• You experience irritation from glycolic acid products

Choose both if:

• You want to alternate between deeper exfoliation (glycolic) and gentler maintenance (lactic)
• You have combination skin with different needs in different areas
• You are using a multi-acid formulation that combines both at lower concentrations

For a deeper understanding of what glycolic acid is and how to incorporate it into your routine, explore our fundamentals guides.

Frequently Asked Questions

Is lactic acid just a weaker version of glycolic acid?

No. Lactic acid is a gentler AHA, but "gentle" does not mean "weak." At appropriate concentrations, lactic acid produces meaningful exfoliation and has demonstrated efficacy for photodamage in double-blind RCTs [11]. It also has humectant properties that glycolic acid lacks. Lactic acid is a different tool, not an inferior one.

Can I switch from lactic acid to glycolic acid?

Yes. If you have been using lactic acid and want to try glycolic acid, reduce your lactic acid use to allow a few days of skin recovery, then introduce glycolic acid at the lowest available concentration 2-3 times per week. Gradually increase frequency as tolerated. Your prior AHA experience means your skin has some baseline tolerance, but glycolic acid may still feel more intense at first.

Which is better for acne?

Glycolic acid has stronger evidence for acne treatment, including a large RCT showing efficacy at 10% [9] and demonstrated antibacterial activity against C. acnes [10]. However, if your acne-prone skin is also sensitive, lactic acid may be more tolerable as a daily maintenance exfoliant. For acne specifically, also consider salicylic acid, which penetrates into pores more effectively than either AHA.

Do they have the same pH requirements?

Both glycolic acid and lactic acid follow the same Henderson-Hasselbalch principles - their activity depends on the proportion of free (non-ionized) acid, which is determined by pH in skincare relative to pKa. Glycolic acid has a pKa of 3.83; lactic acid has a pKa of 3.86. The values are so close that pH affects them nearly identically. A product pH of 3.5-4.0 is effective for both [15].

References

1. 1. Sharad J (2013). Glycolic acid peel therapy - a current review. Clin Cosmet Investig Dermatolreview
2. 2. Tang SC, Yang JH (2018). Dual Effects of Alpha-Hydroxy Acids on the Skin. Moleculesreview
3. 3. Fartasch M, Teal J, Menon GK (1997). Mode of action of glycolic acid on human stratum corneum: ultrastructural and functional evaluation of the epidermal barrier. Arch Dermatol Resclinical trial
4. 4. Bernstein EF, Lee J, Brown DB, et al. (2001). Glycolic acid treatment increases type I collagen mRNA and hyaluronic acid content of human skin. Dermatol Surgclinical trial
5. 5. Kim SJ, Park JH, Kim DH, Won YH, Maibach HI (1998). Increased in vivo collagen synthesis and in vitro cell proliferative effect of glycolic acid. Dermatol Surgclinical trial
6. 6. Sci Rep (2025). Comparative efficacy of topical interventions for facial photoaging: a network meta-analysis. Sci Repnetwork meta-analysis
7. 7. Usuki A, Ohashi A, Sato H, et al. (2003). The inhibitory effect of glycolic acid and lactic acid on melanin synthesis in melanoma cells. Exp Dermatolin vitro study
8. 8. Indian J Dermatol (2025). 80% Lactic Acid Peel Versus 50% Glycolic Acid Peel for Melasma: A Randomised Clinical Trial. Indian J DermatolRCT
9. 9. Abels C, Kaszuba A, Michalak I, et al. (2011). A 10% glycolic acid containing oil-in-water emulsion improves mild acne: a randomized double-blind placebo-controlled trial. J Cosmet DermatolRCT
10. 10. Yang AJ, et al. (2020). pH-Dependent Antibacterial Activity of Glycolic Acid: Implications for Anti-Acne Formulations. Sci Repin vitro study
11. 11. Stiller MJ, Bartolone J, Stern R, et al. (1996). Topical 8% glycolic acid and 8% L-lactic acid creams for the treatment of photodamaged skin. A double-blind vehicle-controlled clinical trial. Arch DermatolRCT
12. 12. Burns RL, Prevost-Blank PL, Lawry MA, et al. (1997). Glycolic acid peels for postinflammatory hyperpigmentation in black patients. A comparative study. Dermatol Surgclinical trial
13. 13. Andersen FA (1998). Final Report on the Safety Assessment of Glycolic Acid. Int J Toxicolsafety assessment
14. 14. Scientific Committee on Consumer Safety (SCCS) (2000). Opinion on Alpha-Hydroxy Acids. SCCNFP/0370/00regulatory opinion
15. 15. Decker LC, Graber EM (1996). Clinical and histological effects of glycolic acid at different concentrations and pH levels. Dermatol Surgclinical trial
16. 16. Kaidbey K, Sutherland B, Bennett P, et al. (2003). Topical glycolic acid enhances photodamage by ultraviolet light. Photodermatol Photoimmunol Photomedclinical trial
17. 17. Kornhauser A, Wei RR, Yamaguchi Y, et al. (2009). The effects of topically applied glycolic acid and salicylic acid on ultraviolet radiation-induced erythema, DNA damage and sunburn cell formation in human skin. J Dermatol Sciclinical trial

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