I have a vitamin C serum that I bought about two months ago at a cosmetics store, and I suddenly noticed the liquid changed color - now it's more yellowish/brown. Is it still effective or is it already damaged? Is there a way to prevent it from oxidizing so quickly?
The problem is that once it turns brown, you've basically wasted your money on something that won't work - oxidation breaks down the active ingredients and that color is proof the product is dead. For next time, keep the bottle in a dark, cool place (the fridge is better if you can), keep the cap tightly closed after each use, and if it comes in a dropper bottle, avoid leaving the cap open while you're using it.
Vitamin C oxidizes with air and light, so that color change means it's already lost its effectiveness and you should stop using it. Once it turns brown, the product is degraded and it can irritate your skin without giving you the benefits you were expecting. For next time, keep the bottle in a dark, cool place (fridge is ideal), keep it tightly closed between uses, and make sure it comes in an opaque container or dark glass, not transparent.
You're right that it's oxidized and lost its potency, but there's an important detail nobody mentions: some brown serums are still *safe* to use, just ineffective, while others can irritate your skin depending on what byproducts formed during oxidation. The most practical thing is to toss it and start fresh, but for your next purchase, keep it in the fridge from day one (not joking, it makes a brutal difference), in a dark bottle sealed tight, and use it within a month or month and a half max. Vitamin C is fragile, so that brownish tint is basically telling you the money's already gone.
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