Lactic acid is one of the most commonly used organic acids on the skin. It was originally derived from milk or plant-derived sugars and has gentle exfoliating properties which make it suitable to be used in peels, serums, masks, and toners. Additionally, it also hydrates the skin. 

It is formulated in concentrations from 2% to 50%; but due to its large molecular size, it doesn’t penetrate much to the deeper layers of the dermis like the other alpha-hydroxy acids (AHAs), such as glycolic acid. It can be used in sensitive skin or darker skin types.

Topical lactic acid is used for the treatment of hyperpigmentation, textural abnormalities, acne scars, enlarged pores, and acne. It has mild peeling effects, but can effectively treat xerotic skin because of its function as a humectant, drawing moisture into the stratum corneum. 

It also decreases melanogenesis and is a gentle treatment for skin hyperpigmentation, particularly in skin of color. However, some side effects like peeling, stinging, erythema, photosensitivity, and hyperpigmentation can occur when improperly used.

Lactic acid, when used daily on the skin in 10%-15% concentration, as a mild exfoliant and humectant can improve texture, decrease pigmentation and improve fine lines without thinning of the skin.

Ammonium lactate is now been widely used in moisturizers, which is the combination of ammonium hydroxide and lactic acid, or the salt of lactic acid. A comparative study demonstrated 12% ammonium lactate to be significantly more effective at reducing xerosis than 5% lactic acid. It is often used for treating keratosis pilaris, calluses, xerosis, and ichthyosis.

Lactic acid still struggles for fame, unlike glycolic acid. But in clinical terms, it is a much safer acid to use, associated with the additional benefit of increasing hydration of the skin which is crucial in its long-term use for photoaging as well as the prevention of wrinkles. 

Source: Talakoub L. Acid series: Lactic acid. Aesthetic dermatology update, September 17, 2021