How to know when your vitamin C serum has oxidised or expired

From its ability to reduce the appearance of uneven skin tone, blemish scarring and dark spots, and the way it protects skin from the free radical damage that comes from UV exposure and pollution (plus, its role in enhancing the skin’s collagen and elastin production process), there’s a whole host of skin-loving benefits. Let’s also not forget its general awesomeness at boosting skin’s overall brightness and glow. No huge surprise, then, why a daily dose of vitamin C is a must in many skincare regimens.

We’re taking you through the tell-tale signs of vitamin C oxidation, what that means in terms of efficacy and how to choose and store your serum for a full and potent shelf life (and super glowy skin!).

More often than not, air, light and heat are the key culprits of vitamin C oxidation. Notoriously difficult to formulate, vitamin C is typically unstable in water solutions, making serums with the ingredient particularly sensitive to the elements. Dark or opaque bottles (protecting from light), and droppers rather than pumps (protecting from air) help to increase on-shelf stability.

Cool, dark and consistent, your fridge is actually the best place to store your vitamin C serums — just don’t mistake them for salad dressing.

The easiest way to tell if your vitamin C serum has oxidised (and therefore lost most of its potency) is by its colour.

Many serums start yellow, some clear. If you’ve noticed that yours has developed into a much darker yellow, a deep amber, or even brown, this may be a sign of oxidation and can result in your serum no longer working as intended.

In the case of many serums, this colour-changing is inevitable. The time it takes to get there, however, will depend on how it has been formulated, packaged, or the way you have used and stored the serum. Our tip? Don’t wait too long after purchasing before using your new vitamin C serum.

If you haven’t noticed colour changes, you might notice a stronger, smokier scent or a thicker consistency. These may come alongside colour changes too and are also signs of oxidation.

It is unlikely that an oxidised or expired vitamin C serum will cause any serious harm to your skin, and the best case scenario is that it won’t offer any benefits and you’ve just wasted time applying it. However, there is a slight chance that using oxidised vitamin C may cause sensitivity (but this sensitivity may also be caused by the vitamin C oxidising on the surface of your skin, when it comes into contact with oxygen).

Bottom line: don’t stress if you’re reading this and only just realising your serum may not be in its best shape. It just means that you won’t be reaping its full brightening, evening, firming, and glow-inducing benefits, and that it’s time for a new bottle.

Free from water, vitamin C powders have superior stability.

In order for your skin to benefit from vitamin C, it needs to be in a form called ascorbic acid. As we’ve mentioned, vitamin C, or ascorbic acid, is unstable and begins to degrade when exposed to heat, oxygen, or light. The ascorbic acid in powdered vitamin C is more resistant than other forms.

A couple of our favourites: goop’s GOOPGLOW 20% Vitamin C and Hyaluronic Acid Glow Serum and Drunk Elephant’s C-Firma™ Fresh Day Serum, both of which require you to mix an L-ascorbic acid powder into a hydrating serum base. This means the shelf life of your serum (about six months) starts on your first day of use, rather than the day it got bottled, and eliminates the risk of early oxidation due to factors like transportation, storage, or sitting in a steamy bathroom. It also ensures that your skin gets the most active, potent dose of vitamin C from the get-go!