Keeping Your Serum Fresh
Close the lid tightly after each use. Since oxygen breaks down vitamin C, you should make sure you close the lid tightly each time you use it, and try to limit how long you leave the bottle open.
Store your vitamin C serum in the fridge. Vitamin C has an extremely short shelf life because it oxidizes, or breaks down when it’s exposed to oxygen. Your fridge is a great place to store vitamin C serum, because the refrigeration will help delay the oxidation process longer than storing it at room temperature. If storing your serum in the refrigerator isn’t an option, find a cool, dark place in your bedroom or another room where you can keep it.
Never store vitamin C serum in the bathroom. The fluctuating heat and humidity in your bathroom will cause your vitamin C serum to break down even more quickly than in other rooms. Try keeping a handheld mirror near the place where you store your vitamin C serum so you can apply it there. If you apply your vitamin C serum in the bathroom, try to find a trick to remind yourself to put it back after you're finished. For instance, you might want to hold the bottle the whole time you are applying the serum instead of sitting it on the counter.
Transfer your serum into smaller opaque containers to help it last longer. Instead of storing your vitamin C serum in a large container, purchase or repurpose smaller opaque glass bottles. Split the serum between these bottles. This will effectively prevent half of your serum from being exposed to oxygen, helping it to last longer.
Discard your serum once it turns yellow or brown. As vitamin C serum oxidizes, it will change colors. Once your serum turns yellow, red, or brown, it has become oxidized and will no longer be effective. For most formulas, this usually happens after about 3 months at room temperature or 5 months with refrigeration, although the exact amount of time will vary between brands.
Choosing a Stable Serum
Avoid choosing a serum which uses water, as it will break down faster. Vitamin C starts to degrade as soon as it comes into contact with water. This process can be slowed down by the addition of preservatives, but the balance must be precise and the formula will still have a shorter shelf life than a formula which does not use water. Look for serums made with ascorbic acid (AA), tetrahexyldecyl ascorbate (THDA), magnesium ascorbyl phosphate (MAP), or sodium ascorbyl phosphate (SAP). You can find options online here.
Choose less powerful but more stable forms of vitamin C. The most common form of vitamin C in skincare is L-ascorbic acid. Unfortunately, this is also one of the least stable forms of the vitamin. Other forms may offer less potency, but increased shelf stability. Look for formulas made with ascorbyl glucoside, magnesium ascorbyl phosphate, and tetrahexyldecyl ascorbate.
Look for a serum in an opaque, airtight tube or bottle. The more your serum is exposed to light and air, the faster it will break down. If you purchase a vitamin C serum in a transparent bottle or a tube which is not airtight, it will probably lose its potency before you can use it all. If you can only find transparent bottles, transfer your new serum to an opaque bottle after you get home.
Purchase small bottles of vitamin C serum so you don’t waste any. To avoid wasting large amounts of serum, try purchasing smaller bottles. You can also check to see if you can find sample sizes of a serum you want to try so you don’t spend a lot of money on a product that will go bad before you use it all. Since vitamin C serum expires after a few months, only buy what you think will use before the expiration date.
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