Obagi Professional-C Serum 20% uses stabilized ascorbic acid - the only form of Vitamin C that can be effectively absorbed by the skin. It provides superior antioxidant protection with deep penetration to the dermal layer. The 20% L-ascorbic acid is the highest concentration available and helps in defending against damaging UVA and UVB rays, helps prevent premature signs of aging such as fine lines and wrinkles, and contributes to collagen synthesis. Obagi Professional-C Serum 20% is suitable for all skin types. Obagi Professional-C Serum is available in four concentrations, for all skin types: * 20% - For the highest concentration on L-Ascorbic Acid * 15% - For all skin types * 10% - For dry, sensitive or reactive skin Directions for use: Apply 5 to 7 drops to the face, neck and chest using fingertips. Apply to cleansed skin in the morning before sunscreen and makeup application. A slight tingling may occur in some people, which diminishes with continued use