Look, I have absolutely nothing against vitamin C serums, but they do come with one major gripe: Many formulas are super unstable, namely ones containing L-ascorbic acid, which is the most pure and effective form of vitamin C.
“Ascorbic acid is water-soluble and is not very stable in solution, but can be best stabilized at a lower pH—this acidity can be irritating to sensitive skin,” says board-certified dermatologist Hadley King, M.D. That’s why experts recommend finding a formula with a concentration of 10% ascorbic acid or lower if you have easily irritated skin. Board-certified dermatologist Geeta Yadav, M.D., founder of FACET Dermatology, seconds the warning: “This ingredient performs best in formulas at a pH of 4 or below, but those with sensitive skin types do better with formulas that are closer to the skin’s natural pH levels, around 6,” she tells mbg.
However! As the sourcing and technology become more and more innovative, brands have figured out how to weave in different forms at clinically effective levels with zero irritation. For example, tetrahexyldecyl ascorbate (also known as THD ascorbate) is an oil-soluble version of the antioxidant and most commonly used in facial oils and moisturizers—so you can have your vitamin C and reap hydration benefits, too.
“A vitamin C moisturizer is a great way to add the ingredient into your routine,” Yadav notes. “Vitamin C is known for brightening skin—but if skin is dehydrated, it’ll maintain a dull appearance. By combining the hydrating and nourishing benefits of a moisturizer with the brightening and free-radical fighting benefits of vitamin C, you’ll get the best of both worlds.”
And because moisturizers typically contain more stable forms of vitamin C, you can layer on your hydrating serums without fear that the brightening antioxidant won’t do its job. In fact, “Some studies have suggested that the fatty acid component of THD ascorbate may allow better penetration into the skin,” King says, thanks to the ingredient’s oil-soluble ability.