I’m using Klairs (a Korean brand). Klairs

I keep my Klairs in the fridge, but it is pretty stable compared to others I have used. I’ve had it for a couple of months and still no discoloration. It makes my skin super soft and smooth. Be sure to use sunscreen with this product!

Vitamin c works best on bare skin. There are a lot of vitamin c products but they are not all the same!

Read the ingredients list, you want “ascorbic acid”. Ascorbic acid is the same as what is in our skin and only this derivative can readily absorb into the skin. All others are more stable, but may not absorb or convert to a product that our skin can use! The vitamin C that is in the product you are using is sodium ascorbyl acetate, which is stable but less effective than ascorbic acid and may not convert into ascorbic acid (no data available) Source: What is the best kind of Vitamin C for skin? (the beauty brains)