If you know anything about skincare products, you know the benefits of a vitamin C serum. It’s a potent antioxidant that protects against the cellular damage caused by pollution and sun exposure. It also brightens skin and helps fade dark spots over time. And it builds collagen, which is a lovely attribute if you like your face to be plump and bouncy.
But there’s a hitch—isn’t there always? Vitamin C is unstable and loses potency when it’s exposed to light or oxygen. Our science guy explains how to find a vitamin C that works and avoid those that fall apart under pressure. —Linda Wells
Vitamin C works because it’s an antioxidant, which means it protects the skin from being oxidized. That’s how all antioxidants work to an extent. Vitamin C in particular is more unstable than most antioxidants because it readily gives up its electrons—oxidation is loss of electrons—and in doing so, it’s more prone to oxidation from the free radicals in our environment. The air is full of them.
You may notice that your vitamin C serum turns from yellow to brown over time. A lot of them are already brown when you open them. That’s a sign that the formula has reacted with oxygen, sunlight, or heat and oxidized and is already breaking down.
A vitamin C serum should last three months after you open it, but that’s if its stored under ideal circumstances: in a well-sealed dark bottle that blocks sunlight and away from heat.
Once the liquid turns brown, it’s not harmful, and studies show that it’s still effective, but just you’re not getting what you’re paying for.
There are so many different versions of vitamin C. There’s ascorbyl glucoside (also known as ascorboglucoside) and tetrahexyldecyl ascorbate (THD ascorbate), both of which are shelf stable. The ascorbic acid form of vitamin C is the most likely form to break down but it’s the best because it’s more bioavailable than the others. Once you open a bottle, just be sure to use it daily before it loses its potency.
Javon Ford is a Los Angeles–based cosmetic chemist. He posts highly informative lessons, reviews, and myth-busting content on TikTok and Instagram @javonford16
