It's okay if you don't mix it. in the same formula The reason is because the reaction time takes time. It doesn't happen immediately. in case of Perfect-C Niacinamide (when compared to ascorbic acid Niacinamide which happens much faster)and if used with another serum which contains Niacinamide will there be any problem because I am afraid of interaction between Niacinamide ? Niacinamide and Ascorbyl Tetraisopalmitate