Why does vitamin C serum smell bad?

What should you check before using it again?

1. 1Check the exact product name and its storage directions. Ask the manufacturer whether that formula's smell is expected, especially if the scent has changed since opening.
2. 2Compare the color with how the serum looked when new. Yellowing can accompany oxidation in an initially almost colorless ascorbic acid serum; that does not make yellow a universal discard rule for every formula.
3. 3Check when you opened it and the label's use period. Do not assume every vitamin C serum lasts the same length of time.
4. 4Review where you kept it. Vitamin C is sensitive to sun, heat and light, so a bottle left in those conditions deserves closer attention.

Does vitamin C serum go bad?

Yes. Vitamin C products lose effectiveness over time, and light can inactivate the vitamin C. General guidance puts their shelf life at about six months, but a particular product can have a much shorter period after opening. Follow its own directions rather than treating six months as a deadline that applies to every bottle. Lower cost vitamin C products can be as effective as pricier versions, so the receipt is not a freshness certificate.4

How should you store vitamin C serum?

Choose protective packaging: dark or tinted glass, or an opaque container, rather than a clear bottle or a jar with a wide opening. An opaque container keeps the contents out of view, which is an unusually useful feature for something sold on appearances. Keep the serum away from heat and direct sunlight, and check whether its label specifically requires refrigeration.24

For example, COSRX The Vitamin C 23 requires refrigeration at 3 to 10 degrees Celsius and upright storage once opened. COSRX recommends using it within two months of opening because it discolors easily through oxidation. The brand also explains that this product can generate gas and make a sound when opened. Do not shake it before opening, because it can overflow; open it first, then shake lightly from left to right. These are directions for that product, not every vitamin C serum.3

What if the serum also irritates your skin?

Vitamin C serum can cause dryness, itching or redness, even apart from questions about freshness. Pause use if those symptoms develop. For sensitive skin, start with a low concentration of ascorbic acid. A stronger formula is not the solution to a bottle whose smell you are trying to identify.2