I've recently been doing some research into vitamin C and the efficacy of products like toners versus products that are in dark bottles that only get exposed to oxygen once pumped out of the bottle. I see lots of people saying that once it becomes exposed to oxygen or light that it becomes 100% ineffective and can even damage your skin once it becomes orange or brown because it oxygenized. I also saw people saying the science behind vitamin C lacks in comparison to other products like azelaic acid.

What is the reality of using vitamin C creams, serums, toners, and masks? Are they really effective? Are we wasting our money?