I have used 2 bottles of the upgraded OST Vitamin C 21.5 serum before using this C20. And thats probably the reason why I am not sold on this one eventhough this is way cheaper. I believe both yield the same results but this is rather sticky and it makes me want to wash my face before applying other skincare on top.  I wait at least 20 minutes before I apply anything else and yet with C20 even after 20 minutes or more it still leaves a sticky feel. I am going back to 21.5 for sure