I started using vitamin c with this product, (I Know it's a bit too much as a beginner.) For first 7/10 days it worked pretty well, I experienced no issue using this rather I think my pigmentation faded. But after that I realized my face is turning horribly red right after using this (I never experienced that before). Had closed comedones around my chin area plus huge red pimples all over my face.
Moreover this product oxidized within 2 weeks (I kept in cool and dark place but not in refrigerator)
Now after a month I left with huge acne scars.

Ocupé este producto por mas de 15 días, ya que leí que era excelente con las manchas del rostro. Quería deshacerme de mis manchas de acné, y fue el peor error, tengo una purga en mi frente, mentón y sobre el labio, horribles, me salieron cientos de espinillas, y mi piel se puso horrible en cuanto a textura, se comenzó a manchar mas aún, y un zarpullido en mi frente como si todos esos granos quisieran explotar.
Mi piel grasa, genera mas grasa con este producto, por las noches es como si hiciera mantequilla, despierto con la piel muy pegajosa, aunque ocupe muy poca cantidad.
Mi piel sensible comenzó a despellejarse en la zona de la nariz, y me generó una picazón insoportable la primera semana, al paso de los días seguía picando pero un poco menos, creí que era acostumbramiento, y que el acné bajaría, pero no, cada día me crecen mas manchas y nuevos granos en mi rostro. Me siento sumamente triste por haber ocupado este producto. No lo volvería a comprar.

I feel that it has helped in my routine, but I thought it would have faster results.
I have not had a bad reaction or anything I can complain about, but since it is my first experience with vitamin C, I may not know the correct results so well.
But so far, I've been using it for a few weeks and it's helping in the routine, my skin absorbs quickly and the smell is fake orange juice, it does not bother me too much.
When I finish I will research on other brands and see the long term result to decide if I continue with this or change of brand.

i used to use and love the C20 serum and i thought that since they have just rebranded, i would feel the same about this product. i am extremely disappointed that this has caused me severe breakout, several weeks before my wedding! i can't stress this enough-this product made my life miserable. i have clear skin and i only get pimples from time to time. but after using this, it made me break out so much. pimples that have puss in them, which i never had before. i dont know which ingredient made me breakout. will never recommend this product. such a disaster.

I've tried both OST C20 and C21.5 and was satisfied with how they made PIH disappear (C20 results were better!). I wanted to try this version but got a severe allergic reaction, my face became red, swollen and was itching so bad (I didn't patch test because I didn't think it'd happen, stupid me!). Make sure to PATCH TEST!! I don't think that I even have a sensitive skin but please make sure to patch test (wow!I said it 3 times!).
P.S. I just noticed some other reviewer had a similar reaction. I'm trying to figure out which ingredient caused it so, If you have any idea, please tell us orz
I gave it 1 star because of the allergic reaction. It may work for you, tough!

I bought this serum for its promise related to faster effects and that it'd have more stability than its successor, but the truth is that it has turned out to be very very bad. The smell is similar to raw meat, and once it opens it begins to bubble as if it were soda. It absorbs fairly quickly and leaves no sticky sensation as its previous version, but I wouldn't recommend it without performing the patch test first. My skin had a severe reaction and it became red, swollen and itchy. Even when I use it with other serum or toners, it feels hot and my skin turns red. Please be very careful if you're going to try it, especially if you have sensitive skin. I'm still trying to find out why this reaction is caused, there are already several similar cases among my relatives.