Dr. Dennis Gross observed in his practice that many patients were experiencing vitamin D deficiencies as they protected themselves from exposure to the sun’s UV rays. Therefore, he set out to provide sufficient levels of vitamin D through topical application, according to the company. Though difficult to manufacture, this product contains the ingredient ergocalciferol, denoting that the vitamin is active. Dr. Dennis Gross Skincare Active Vitamin D Serum-Oil ($65), new at Sephora, is a s...