Researched by Helena DeMaria-Williams
Formula IngredientpH AdjusterINCI: Sodium Hydroxide
Sodium hydroxide (also known as lye or caustic soda) is a highly alkaline inorganic compound used in cosmetics in very small amounts to adjust the pH of formulations. In its pure form it is a potent skin irritant, but it reacts fully with acids during manufacturing — meaning no free sodium hydroxide remains in the finished product. It is particularly important in exfoliant and cleanser formulations where maintaining a precise pH (3–4 for AHAs, 4.5–6 for cleansers) is critical to both safety and efficacy.
Category
pH Adjuster
Irritation Risk
Low
Skin Types
all skin types, oily, dry, combination, sensitive, acne-prone
pH balancing, formulation stabilisation, ensures active ingredients work at correct pH, enables effective exfoliants and cleansers
Pure sodium hydroxide is a potent skin irritant and sensitiser at concentrations of 10% or higher. However, in finished cosmetic products it is fully neutralised during formulation and poses minimal irritation risk. EU Annex III restricts its use: pH must not exceed 11 in most cosmetic applications, up to 12.7 in depilatories, and concentration is capped at 2% general / 4.5% professional in hair straighteners and 5% in nail cuticle solvents. In typical leave-on skincare, used amounts are well below 1%.
Sodium hydroxide is a pH adjuster with no photosensitising properties. No sun protection precautions are required based on its presence in a formulation.
Limited specific data is available for topical cosmetic use during pregnancy. The Cosmetic Ingredient Review (CIR) panel considers inorganic hydroxides safe when formulated to be non-irritating. Given that no free sodium hydroxide remains in finished products and typical use levels are below 1%, topical exposure is considered low risk, but specific guidance from a healthcare provider is advisable given limited reproductive safety data (Motherfigure / CIR assessment).
Sodium hydroxide (also known as lye or caustic soda) is a highly alkaline inorganic compound used in cosmetics in very small amounts to adjust the pH of formulations. In its pure form it is a potent skin irritant, but it reacts fully with acids during manufacturing — meaning no free sodium hydroxide remains in the finished product. It is particularly important in exfoliant and cleanser formulations where maintaining a precise pH (3–4 for AHAs, 4.5–6 for cleansers) is critical to both safety and efficacy.
Individuals with known alkali sensitivity (rare); avoid high-concentration professional-grade products without trained supervision should review the considerations above before use.