Vitamin C is a star ingredient in countless dermatologists, estheticians and skincare lovers’ books. It’s a powerful antioxidant that helps protect skin from free radical damage, and as a result, prevents premature signs of aging from forming. Vitamin C (known as L-ascorbic acid in its pure form) is also known to fade dark spots and even out skin tone and texture for a brighter complexion. Our only complaint about vitamin C is that it’s unstable; this means it can degrade and oxidize over time and lose efficacy. Factors like light, air, heat and humidity can all speed up the oxidation process. But how can you tell if your vitamin C serum has gone bad? We asked Dr. Elizabeth Houshmad, a board-certified dermatologist based in Dallas to find out. 
