On that note, you can find ascorbic acid in three different formats: water-based serums, powders and anhydrous suspensions, all of which have their pros and cons (illustrated above).
To quickly recap, the main pros of a water-based serum is that it's the OG, and the most tested with clinical studies. However, they have the shortest shelf-life of the three. Powders are the most stable, but they can be messy, and anhydrous suspensions offer more stability than a water-based serum, but can have inconsistent formulas (some have a gritty texture, which should be avoided).
Of the three, “water-based serums are the classic way to go,” says Fu, who adds that you want to store these serums in vampire settings (i.e., in cold, dark places) and be mindful of their shelf life.
Regardless of the percentage or format you choose, remember that consistency is key. Vitamin C should be used every morning, after cleansing and before any moisturizer or sunscreen.