As essential as vitamin C is to your skin care diet, not all vitamin C serums are created equal. According to Dr. Magovern, there are four main factors you need to consider when looking for a vitamin C serum:

Form of Vitamin C. Vitamin C comes in different forms—L-ascorbic acid, tetrahexyldecyl ascorbate, ascorbyl palmitate and sodium ascorbyl phosphate, among others—but not all of them are equal in terms of stability and absorption rate. “As with all active ingredients in skin care and critical with vitamin C, it is essential that it is well formulated and has good bioavailability and penetration,” explains Dr. Magovern. “In other words, it is able to get to where it needs to go and do what it’s supposed to do. Most serums on the market contain vitamin C in the form of L-ascorbic acid, but some experts prefer the ester form, called tetrahexyldecyl ascorbate, which they believe can be more effective.”

Concentration. Concentration is basically the strength of the acid, explains Dr. Magovern. “A concentration of at least 10% L-ascorbic acid is recommended; however, you will find some products on the market that offer 15 or 20%. The concentration is important, but the best vitamin C serums combine other antioxidants and boosters like vitamin E and ferulic acid, which probably adds more value than just increasing the concentration alone.”

Formulation. While vitamin C alone has proven benefits, studies show that combining it with equally potent ingredients can heighten its efficacy.