What to look for in a vitamin C serum depends on your personal skin concerns. If you're concerned about acne, be sure to look for products that are oil-free. "Vitamin C is often formulated with vitamin E for their synergistic antioxidant effects, but vitamin E is an oil and may cause flares on acne prone skin," says Love.
Keep in mind that vitamin C serums can be irritating to the skin. If you're concerned about sensitivity, look for brands that advertise "sensitive skin" or note that the product is formulated to reduce irritation when you shop for products.
Sometimes it's OK to buy skin care products and let them sit on the shelf for a few months before using them. But you don't want to do that with products that contain vitamin C. "L-ascorbic acid [vitamin C] is difficult to stabilize, which is why a 'steal' is probably too good to be true. Both air and sunlight can oxidize vitamin C, causing it to lose some of its efficacy," says Love.
Besides using the product as quickly as you can, Love also suggests opting for a serum that comes in a container that blocks out light. "Minimize the amount of time the product is exposed to air by applying it quickly to your hands and screwing the top back on the bottle securely. Some products now come in containers that don't require opening the top, so this is also helpful," says Love.