Lactic Acid (AHA)

How it works

Lactic acid (2-hydroxypropanoic acid) is a water-soluble alpha-hydroxy acid (AHA) that disrupts corneocyte cohesion in the stratum corneum. It weakens desmosomal proteins, such as corneodesmosin, by chelating calcium ions, which accelerates desquamation. Its molecular weight (90.08 g/mol) is larger than glycolic acid (76.05 g/mol), limiting penetration into deeper skin layers and reducing irritation. Lactic acid also stimulates ceramide synthesis by upregulating serine palmitoyltransferase, enhancing barrier function. Its humectant properties arise from hydroxyl and carboxyl groups that bind water molecules, increasing epidermal hydration. Additionally, it modulates keratinocyte differentiation, promoting a smoother skin surface.

Clinical evidence

A 2001 study by Wang et al. (Archives of Dermatological Research) demonstrated that lactic acid improves skin hydration and barrier function at concentrations of 5-12%. The Cosmetic Ingredient Review (CIR) Expert Panel (2016) concluded that lactic acid is safe for use in cosmetics at concentrations up to 10% (pH ≥ 3.5), with higher concentrations requiring professional supervision. A 2017 study by Moghimipour (Journal of Cosmetic Dermatology) found that lactic acid at 5% significantly reduced hyperpigmentation in melasma patients after 8 weeks. The EU Scientific Committee on Consumer Safety (SCCS) supports its use in leave-on products at ≤10% (pH ≥ 3.5). DermNet NZ confirms its efficacy for keratosis pilaris and mild acne at 5-10% concentrations.

Dosing and protocol

For at-home use, lactic acid concentrations typically range from 5% to 10%, with a pH between 3.5 and 4.0 for optimal efficacy and minimal irritation. The EU Scientific Committee on Consumer Safety (SCCS) recommends a maximum concentration of 10% for leave-on products (pH ≥ 3.5). Begin with once-weekly application, gradually increasing to 2-3 times per week if tolerated. Apply to dry skin in the evening, avoiding the eye area. Follow with a moisturizer to mitigate dryness. Due to its photosensitizing effects, daily SPF 30+ is essential. Professional peels may use concentrations up to 30%, but these require dermatological supervision.

Interactions with other actives

Lactic acid pairs well with hydrating ingredients like hyaluronic acid and glycerin to counteract dryness. Avoid combining it with other strong exfoliants (e.g., glycolic acid, salicylic acid) in the same routine to prevent over-exfoliation. Retinoids can be used on alternate nights, but introduce them gradually. Vitamin C (L-ascorbic acid) can be layered if the pH of both products is compatible (lactic acid's pH should be ≥ 3.5). Avoid using lactic acid immediately after shaving or waxing to minimize irritation.

Common mistakes

A common mistake is overusing lactic acid, especially when starting out. Applying it daily or using high concentrations (e.g., 10%) too soon can lead to redness, peeling, or a compromised skin barrier. Another error is skipping SPF, which increases sun damage risk. Some users also layer lactic acid with incompatible actives (e.g., vitamin C at pH < 3.5 or benzoyl peroxide), causing irritation. Always patch-test new products and introduce them gradually to assess tolerance.

FAQ

Can I use lactic acid if I have rosacea or eczema?

Lactic acid may be tolerated by some individuals with rosacea or eczema, but it depends on skin sensitivity. Its hydrating properties can benefit eczema-prone skin, but the low pH may trigger stinging in rosacea. Start with a low concentration (e.g., 2-3%) and a higher pH (4.0-4.5), or opt for lactobionic acid, a gentler polyhydroxy acid. Consult a dermatologist before use if you have these conditions.

How long does it take to see results with lactic acid?

Visible improvements in skin texture and brightness typically appear after 4-6 weeks of consistent use, as lactic acid gradually renews the skin's surface. For hyperpigmentation, results may take 8-12 weeks due to slower cellular turnover. Moghimipour (2017) noted significant improvements in melasma after 8 weeks of 5% lactic acid use. Consistency and sun protection are key to achieving results.

Is lactic acid safe during pregnancy or breastfeeding?

Lactic acid is generally considered safe for use during pregnancy and breastfeeding at concentrations ≤10%, as it is not significantly absorbed systemically. The American College of Obstetricians and Gynecologists (ACOG) and the NHS classify topical AHAs as low-risk during pregnancy. However, avoid professional-grade peels or high concentrations (e.g., >10%) without medical supervision. Always consult your healthcare provider before use.

Sources

• Wang et al. Lactic acid and skin hydration. Archives of Dermatological Research 2001
• Cosmetic Ingredient Review Expert Panel. Safety assessment of alpha-hydroxy acids. CIR 2016
• Moghimipour E. Lactic acid in melasma treatment. Journal of Cosmetic Dermatology 2017
• EU Scientific Committee on Consumer Safety. Opinion on alpha-hydroxy acids. SCCS 2016
• DermNet NZ. Lactic acid for keratosis pilaris. DermNet NZ 2023