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 vitamin C penetrates more deeply than other forms, making it more likely to reach the skin layers where you actually need it to work. Another form, Tetrahexyldecyl ascorbate, 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 and can oxidize in the presence of 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. That being said, it’s important to store your vitamin C serum in a cool, dark place.
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