Despite its efficacy, vitamin C is notoriously unstable and can quickly go from a hyperpigmentation-zapping wonder ingredient to a $60 bottle of smelly serum ... closer to 3 or 4 percent.

What does a vitamin C serum do? 'Vitamin C is a staple for so many skin concerns,' explains facialist Abigail James. 'It's an antioxidant, which means it can protect against free radical damage ...