Why Does Vitamin C Serum Turn Brown, and Can You Still Use It?

Your vitamin C serum was almost clear when you opened it. Now it is yellow, amber or the colour of weak tea.

If the ingredient list says ascorbic acid, that darkening usually means the L-ascorbic acid is degrading. The colour confirms that the formula has changed, but it gives you no concentration measurement.

So, can you continue using it?

If the serum began clear or pale yellow and has become distinctly amber or brown, I would replace it. It may still contain some L-ascorbic acid, but you no longer know whether you are applying anything close to the concentration stated on the label.

First, check which vitamin C you are using

The familiar clear-to-brown colour change mainly concerns L-ascorbic acid. You will usually find it on the ingredient list as ascorbic acid.

L-ascorbic acid is the form used directly in vitamin-C-dependent reactions in skin. It is also difficult to keep stable, particularly in a water-based serum that is opened every day.

Ascorbyl glucoside, sodium ascorbyl phosphate, magnesium ascorbyl phosphate and tetrahexyldecyl ascorbate are vitamin C derivatives. Their chemical structures differ from L-ascorbic acid, so their stability and colour changes differ as well. Skin delivery and conversion also need to be considered separately for each derivative.

Do not judge a derivative serum with a colour guide written for L-ascorbic acid. The formula may also be yellow, orange or brown from the day you open it because it contains ferulic acid, plant extracts or oils.

If you are unsure what your ingredient list means, I explain how the forms differ in Vitamin C for Skin: What Works and What Fails.

Why does L-ascorbic acid turn brown?

Vitamin C can donate electrons during oxidation-reduction reactions. On your skin, this allows it to react with some reactive oxygen species before they react with skin lipids, proteins or DNA. You can read more about that process in Do Antioxidant Serums Work for Oxidative Stress in Skin?.

The same willingness to react makes L-ascorbic acid difficult to keep unchanged inside a bottle.

When L-ascorbic acid is exposed to oxygen, it can first oxidise to dehydroascorbic acid. Further reactions can then break it down into other compounds. Some later products and reactions contribute to the yellow, orange and brown colours that develop in the serum.

This is not a direct conversion from clear vitamin C into one brown compound. The first oxidation product is not brown, and some L-ascorbic acid can be lost before you notice a large colour change.

By the time a previously pale serum looks like tea, degradation has progressed far enough to change what you can see. The concentration that remains is still unknown.

Research using heated aqueous ascorbic acid solutions helps explain how oxygen and pH affect its degradation. Those experiments used much simpler and more severe conditions than a serum encounters during ordinary storage, so they do not identify the exact mixture inside every brown cosmetic formula (Yuan and Chen, 1998).

When I assess formulation stability, I do not decide that an ingredient has survived by looking at the bottle. I measure its concentration at planned time points with a validated chemical assay. I record colour as well, but colour and concentration answer different questions.

At home, the darkening helps you recognise that the serum has changed. It will never give you the percentage of L-ascorbic acid left in the dropper. Your bathroom cabinet has many functions, but chromatography is not one of them.

How dark is too dark for vitamin C serum?

There is no universal colour at which every serum suddenly becomes ineffective. Formulas begin with different colours, and their other ingredients affect what you see.

Compare your serum with the colour it had when you first opened it.

It is still clear or the same pale yellow

Continue using it within the period stated on the packaging. A pale yellow colour may be normal from the beginning.

The unchanged colour does not prove that every milligram of L-ascorbic acid is still present. It simply gives you no visible reason to suspect extensive degradation.

It has become more yellow

Some degradation may have occurred. Check whether the cap closes properly and whether you have been keeping the bottle near heat or sunlight.

A small colour change gives you no measure of how much vitamin C has been lost. Watch whether the darkening continues and follow the manufacturer’s storage and period-after-opening instructions.

It has become amber, orange-brown or brown

If the serum began clear or pale, replace it.

You do not need to wait until it smells strange, separates or begins to sting. Those changes give you additional reasons to stop, but they are not required before a clearly brown L-ascorbic acid serum becomes an unreliable source of L-ascorbic acid.

This advice changes when the serum was already dark on the first day. In that situation, the colour may come from another ingredient, and later changes may be difficult to judge by eye.

Taking a photograph when you open a new bottle can help you compare the colour under similar lighting. A photograph cannot calculate potency, but it is more dependable than trying to remember exactly which shade of yellow you saw six weeks ago.

Is brown vitamin C serum dangerous?

Brown colour alone does not prove that the serum has become toxic. It shows that chemical changes have occurred and that the original concentration can no longer be assumed.

That is enough reason to replace it. You chose the product for a stated concentration of L-ascorbic acid, while the darkened serum now gives you an unknown dose.

I would not blame every sting on oxidation, either. L-ascorbic acid is often formulated at a low pH because a greater proportion remains uncharged and can pass through the outer skin barrier more readily. In a frequently cited absorption study, the tested L-ascorbic acid solutions penetrated pig skin only when formulated below pH 3.5 (Pinnell et al., 2001).

That acidity can sting sensitive skin or skin with a damaged barrier even when the serum is fresh.

If a serum that your skin previously tolerated begins to cause burning, itching or persistent redness after its colour or texture changes, stop using it. The reaction alone will not identify which ingredient or degradation product caused the problem.

If a new serum stings from its first applications, read Can Sensitive Skin Handle Vitamin C Serum?. The form of vitamin C, the pH of the complete formula and the condition of your barrier all affect what you feel.

Why did my vitamin C serum turn brown so quickly?

How you store the bottle affects the rate of degradation, but the problem may have begun long before the serum reached your bathroom.

In a water-based formula, dissolved molecules can move through the liquid and react. The rate depends on the pH and solvent composition, as well as the oxygen present during manufacture and storage. Trace metal ions may accelerate oxidation.

Opening the bottle introduces fresh air into the headspace. A dropper is removed each time you apply the serum, which can increase repeated air contact. A pump may reduce exposure, although it is not automatically airtight.

Heat generally increases the rate of chemical reactions. Light can accelerate degradation in susceptible formulas. This is why a serum kept beside a sunny window is likely to change faster than the same serum stored somewhere cool and dark.

Packaging helps only when it suits the formula. An opaque bottle reduces light exposure, but it will not compensate for poor chemical stability. A polished airless package also tells you very little about the amount of L-ascorbic acid that will remain after several months.

The manufacturer needs to test the finished serum in the package used for sale. Looking at one ingredient, one antioxidant or the colour of the bottle is not stability testing.

Do vitamin E and ferulic acid stop vitamin C from turning brown?

Vitamin E and ferulic acid can improve a formula when their concentrations and solvent system have been developed together. Their names on an ingredient list do not guarantee that the serum will remain stable.

In one study, 0.5% ferulic acid improved the chemical stability of a solution containing 15% L-ascorbic acid and 1% alpha-tocopherol. The combination also provided greater protection against solar-simulated radiation than the vitamin C and E solution tested without ferulic acid (Lin et al., 2005).

A later study in human skin found that this specific antioxidant solution reduced several responses to ultraviolet exposure (Murray et al., 2008).

These findings do not transfer automatically to every serum listing the same three antioxidants. Concentration affects the formula, while the solvent system and pH affect how the ingredients behave. Packaging and manufacturing conditions also influence how much survives storage.

The ingredient list shows what was included when the serum was made. Stability testing shows how much remains over time.

Vitamin E also has its own role in skin rather than serving only as a supporting ingredient for vitamin C. I explain the differences in Vitamin C vs Vitamin E for Skin.

How can you stop vitamin C serum from turning brown?

Close the bottle as soon as you finish using it. Store it away from direct sunlight and sources of heat.

Do not leave it on a sunny windowsill. A bathroom cabinet may be suitable if the room does not become very hot, although repeated steam and temperature changes are not ideal for an unstable aqueous formula.

Refrigeration may slow degradation in some products, but follow the manufacturer’s instructions. Cold storage can alter the texture or cause ingredients to crystallise when the formula was not designed for it.

Also check the period-after-opening symbol. The printed expiry date usually assumes that the unopened product has been stored correctly. Once you begin opening the package, air exposure and your storage conditions influence what happens next.

Buy a bottle size you can finish within that period. The larger bottle is not the better purchase if the last third turns brown before you apply it.

Can you still use vitamin C serum after it turns brown?

If an L-ascorbic acid serum began clear or pale yellow and is now distinctly amber or brown, replace it.

The serum may not be dangerous, and it may still contain some L-ascorbic acid. Neither point answers the question you need answered: does it still provide the concentration you bought it for? Once the formula has darkened that far, you have no reliable basis for assuming that it does.

A slight yellow tint present from the beginning may be normal. A serum made with a vitamin C derivative also needs to be judged according to that derivative and the complete formula.

Check the ingredient list first. Then compare the product with its original colour. If a pale L-ascorbic acid serum has turned brown, let the bottle go.

If you are replacing it because vitamin C no longer suits what your skin needs, compare vitamin C with hyaluronic acid for hydration or retinol with vitamin C for wrinkles and photoageing.

Talk to you soon,

Dr Bozica

References

Yuan, J. P., and Chen, F. (1998). Degradation of ascorbic acid in aqueous solution. Journal of Agricultural and Food Chemistry, 46(12), 5078–5082.

Pinnell, S. R., Yang, H., Omar, M., Monteiro-Riviere, N., DeBuys, H. V., Walker, L. C., Wang, Y., and Levine, M. (2001). Topical L-ascorbic acid: percutaneous absorption studies. Dermatologic Surgery, 27(2), 137–142.

Lin, F. H., Lin, J. Y., Gupta, R. D., et al. (2005). Ferulic acid stabilizes a solution of vitamins C and E and doubles its photoprotection of skin. Journal of Investigative Dermatology, 125(4), 826–832.

Murray, J. C., Burch, J. A., Streilein, R. D., Iannacchione, M. A., Hall, R. P., and Pinnell, S. R. (2008). A topical antioxidant solution containing vitamins C and E stabilized by ferulic acid provides protection for human skin against damage caused by ultraviolet irradiation. Journal of the American Academy of Dermatology, 59(3), 418–425.