But when it comes to stamping out dullness, C isn't the only game in town. In fact, if you ask some derms, the idea of relying only on vitamin C as a one-and-done solution for brightening may even be... outdated. To start, the ingredient is notoriously difficult to stabilize, which means we may not be getting the illuminating power we think we are. Variables such as light, pH level, temperature, and simply the quality of the formulation can affect how well your C does its job.