While vitamin C is unrivaled as a skin-brightening antioxidant, there is a catch (well, sometimes), and that’s that it’s notoriously unstable. If it breaks down (or oxidizes) before it hits your skin, it’ll be rendered ineffective. What’s more, is that the oxidized vitamin C could potentially cause irritation and blackheads.
“That is the downside of vitamin C,” Fernandez says. “It can be unstable, which means it can be made ineffective by environmental factors (like too much light or heat). For most formulas, the color will darken and it will have an incorrect balance of actives and alcohol.”
So, how do you make sure your vitamin C is stable? Fernandez says all you need to do is check the color and smell of the product. If the color is darker than it was when you bought it (for example, if it turned a dark brown color) or it smells a little funky, toss it. It’s not worth using anymore. The fact is that it’s already oxidized and you simply won’t be getting the benefits you once were. To avoid this from happening, Fernandez recommends making “sure to store in a temperate place with no direct sunlight or heat fluctuations.”