Update: This has been asked so frequently that I decided to do a full post on it! You can read it here.
Can Vitamin C (Ascorbic Acid) and Niacinamide Be Used Together?
A common concern on /r/SkinCareAddiction is the mixing of two incompatible ingredients.
Vitamin C (in the form of ascorbic acid) and niacinamide work best at different pHs. The combination of the two in the same product can cause niacinamide to break down into a form that causes skin flushing.
Applying one product with Vitamin C and then one with niacinamide (and vice-versa) isn’t a problem though.