Seeing multi-ingredient formulations on the market? If you see vitamin C plus an exfoliating acid or other active ingredient in the same product, don’t panic. Instead, take note that some may be formulated this way “to offer synergistic benefits, but these products have been pre-formulated and safety-tested,” says Dr. Zeichner. “When separate products are combined, especially at higher concentrations, they are more likely to cause irritation.”

Ingredients like ferulic acid, hyaluronic acid, vitamin E, and niacinamide also often sit alongside vitamin C in the same bottle to help boost the effects or provide additional hydration.

How do you choose a vitamin C serum?

In skincare products, topical vitamin C comes in two forms: as pure ascorbic acid or in its ester form, explains Dr. Zeichner. “Pure ascorbic acid is more potent but more unstable and potentially irritating,” he says. If your skin has a high tolerance for vitamin C, or you want stronger antioxidant power, look for ascorbic acid or l-ascorbic acid on the label.

Meanwhile, “vitamin C esters like sodium ascorbyl phosphate and other compounds with the word ascorbyl in them are more stable and better tolerated but less potent,” he says. “They need to be converted into the active ascorbic acid by the skin after application.” He recommends dry and sensitive skin types use vitamin C esters.

As much as we love our vitamin C, the skincare ingredient can be a finicky one sometimes. “Pure vitamin C is a very temperamental ingredient that can easily be inactivated,” says Dr. Zeichner. Make sure your vitamin C serum comes in a dark or opaque bottle: “Pure ascorbic acid is particularly sensitive to changes in pH, temperature, and UV light. This is why pure ascorbic acid comes in opaque bottles, as UV will inactivate the molecule.”

As with sunscreen, pay attention to the expiration date on the bottle. “The formulas tend to turn orange as the vitamin C oxidises,” Dr. Zeichner says. “If you have a formula that does not look, smell, or feel as it did when you purchased it, toss it.”

Here are some of the best vitamin C serums we tested: