If you are going to use a vitamin C product, it's best to use one in a form your skin can use with the least amount of irritation. Vitamin C Stability and Effectiveness One of the bad things about Vitamin C is that it oxidizes very easily.When a C serum oxidizes, it turns yellow or orange and looses its effectiveness.f you are going to use a vitamin C product, it's best to use one in a form your skin can use with the least amount of irritation. Vitamin C Stability and Effectiveness One of the bad things about Vitamin C is that it oxidizes very easily.