Ascorbic acid—also known as L-ascorbic acid—has the most research of any form of vitamin C when it comes to benefits for skin. It is a potent antioxidant and skin-soothing agent that can improve the appearance of signs of aging. [1-3]
Ascorbic acid is also a powerhouse when mixed with other antioxidants, or when used alone in higher concentrations, such as 15% or 20% or greater, amounts that can be great for treating an uneven skin tone. [4,5]
A particularly vulnerable antioxidant when exposed to UV light and air, ascorbic acid must be packaged to protect it from these elements during routine use, or its effectiveness will gradually become diminished to the point of not working at all (you will see this as oxidized discoloration, meaning the product turns a copper to brownish color).  For this reason, avoid any vitamin C (ascorbic acid) products packaged in jars.
- Pinnell S, Yang H, Omar M, Monteiro-Riviere N, DeBuys H, Walker L, Wang Y, Levine M. Topical L-ascorbic acid: percutaneous absorption studies. Dermatol Surg. 2001;27(2):137-42.
- Telang P. Vitamin C in dermatology. Indian Dermatol Online J. 2013;4(2):143-146.
- Farris P. Topical vitamin C. Dermatol Surg. 2005;31(7 Pt. 2):814-7; Discussion 818.
- Murray J. A topical antioxidant solution containing vitamins C and E stabilized by ferulic acid... J Am Acad Dermatol. 2008;59(3):418-25.
- Xu T. Split-face study of topical 23.8% L-ascorbic acid serum... J Drugs Dermatol. 2012;11(1):51-6.
- Iqbaul A, Sheraz M, Ahmed, S et. al. Photostability and Interaction of Ascorbic Acid in Cream Formulations. AAPS PharmSciTech. 2011 Sep; 12(3): 917–923.