Ask a Dermatologist: Explain AHAs. What’s the Difference Between Glycolic Acid, Lactic Acid, Tartaric Acid and Malic Acid?

Dr. Courtney Rubin, our Chief Medical Officer, weighs in on your most pressing skincare questions.

While glycolic, lactic, tartartic and malic acids are all alpha hydroxy acids, or AHAs, glycolic acid is the smallest molecule of the bunch, so it is slightly more effective at penetrating the stratum corneum (the very top layer of the epidermis) and working its magic to promote skin turnover and exfoliation. While it is a more powerful and effective exfoliator than lactic acid, glycolic acid is also harsher and more likely to induce redness, irritation and sensitivity. As a larger molecule, lactic acid doesn't penetrate as deeply as glycolic acid, and therefore has a gentler, milder exfoliating action. 

Tartaric and malic acids are even larger molecules in the AHA family (much larger than lactic acid and much, much larger than glycolic acid), so they are less able to penetrate the top layers of skin to exert their effects. You won’t often see tartaric and malic acids featured in skincare because their large size prevents them from effectively exfoliating in the same way that glycolic and lactic acids are able to.