Lactic acid is a type of alpha hydroxy acid (AHA). Alpha hydroxy acids are natural acids found in foods. Lactic acid comes from fermented foods. Alpha hydroxy acids like lactic acid work by removing the top layers of dead skin cells. Lactic acid seems to increase the thickness of deeper layers of skin, promoting firmness. People use lactic acid for dry skin. They also use it for acne, aging skin, canker sores, warts.