The key ingredient in these serums is L-ascorbic acid, a pure form of vitamin C, says Joshua Zeichner, MD, a board-certified dermatologist in New York City. This type of serum penetrates more deeply than other forms, making it more likely to reach the layers of skin where you actually need it to work. Another popular one? Tetrahexyldecyl ascorbate, which tends to be more stable than its counterparts (read:it lasts longer).
Pay attention to the concentration, too. “Look for serums that contain 10% to 20% vitamin C,” says Morgan Rabach, MD, a board-certified dermatologist in New York City. “Less than 10% is not as effective, and over 20% can cause skin irritation.” A supporting cast of active ingredients, such as vitamin E and ferulic acid, can also boost its efficacy. Some vitamin C serums may even boast nice-to-haves including hyaluronic acid, squalane, and other hydrators.
Don’t forget to peep the packaging. Vitamin C is notoriously unstable, so it can oxidize with light and heat. You want darkly tinted glass bottles with airless delivery, since air can also cause pure vitamin C to degrade, says Dr. Rabach.