Lactic acid is an AHA (alpha-hydroxy acid) and gently exfoliates the skin. It is known to reduce the appearance of fine lines, wrinkles and dark spots as well as brighten, soften and smooth the complexion because it promotes cell turnover and renewal. Skin will appear plumper as moisture is retained on the skin. It also stimulates collagen production.
This ingredient is naturally found in milk so if used in skincare formulations they may not be classed as vegan. There are mainly synthetic forms of lactic acid found on the market so it's important to check with the brand where they source their lactic acid from. Also check how high it ranks on the ingredient list depending on your skin type as it can cause dryness, redness and peeling in higher concentrations. Always wear sunscreen following its use which should be limited to once or twice per week so ideal for inclusion in masks and exfoliating treatments.
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Journal of the American Academy of Dermatology, Volume 35, Issue 3, Pt 1, September 1996, Pages 388-391
Dual Effects of Alpha-Hydroxy Acids on the Skin, Molecules 2018, 23