Obagi Professional C Serum 15% 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 15% L-ascorbic acid 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 15% is recommended for normal skin. This product is great for those with aging and sun-damaged skin. It also works well for people who suffer from hyperpigmentation. Directions: Apply 5 to 7 drops of Obagi Professional C Serum 10% 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.