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Vitamin C serums, explained: what the antioxidant claim means, and why some turn orange
Vitamin C is one of the more genuinely evidence-backed actives in skincare, and also one of the least stable — here is what it does, and what a colour change actually tells you.
Petal & Proof Editorial Team4 min read
What vitamin C is doing in a serum
Vitamin C — usually L-ascorbic acid, or a more stable derivative such as sodium ascorbyl phosphate or ascorbyl glucoside — is included in skincare as an antioxidant. Applied topically, it is generally understood to help neutralise the free radicals generated by everyday UV exposure and pollution, which is why it is commonly paired with, rather than used instead of, daily sunscreen. It is also widely used for its role in supporting a brighter, more even-looking complexion over consistent use. Of the 13 face serums in this dataset, three carry a vitamin C tag: Simple Daily Face Serum 100ml (£5.53), The Ordinary Face Serum 15ml (£5.87) and Neutrogena Face Serum 200ml (£11.39) — a genuinely wide spread for the same active, which says more about packaging and brand than about the ingredient itself.
Why some vitamin C serums turn orange
L-ascorbic acid in particular is notably unstable — it degrades when exposed to air and light, a process called oxidation. As it breaks down, the serum's colour shifts from clear or pale yellow towards orange or brown. A serum that has changed colour has lost a meaningful amount of its antioxidant activity, even if it is not harmful to use. This instability is also why more stable derivatives exist: they oxidise more slowly, at some cost to potency compared with fresh L-ascorbic acid, which is the trade-off formulators are making when they choose one form over another.
Concentration is not the whole story
Vitamin C serums typically sit somewhere between around 5% and 20% concentration. Higher is not automatically better: above roughly 15-20%, the added benefit tends to plateau while the risk of stinging or irritation, particularly on sensitive skin, rises. A lower-percentage, well-formulated serum used consistently is generally more useful than a high-percentage one abandoned after a week of irritation. Where a brand states an exact percentage, that is more informative than front-of-bottle marketing language alone.
Format and packaging matter more than for most actives
Because vitamin C oxidises on contact with air and light, packaging is a genuine functional feature rather than a cosmetic detail. Opaque bottles, airless pumps and dark glass all slow the process compared with a clear dropper bottle that is opened repeatedly. Dipping fingers into an open jar introduces more air and contamination with every use; a pump or dropper limits that exposure. None of this fixes an unstable formula outright, but it meaningfully extends how long a serum stays effective once opened.
Where it fits in a routine, and what not to combine it with
Vitamin C is most commonly applied in the morning, after cleansing and before moisturiser and sunscreen, though an evening routine works too. Combining it with retinol in the same application is generally best avoided while your skin adjusts to either ingredient, since both are potent and layering them raises the chance of irritation — many people alternate, using vitamin C in the morning and retinol at night instead. Niacinamide, despite older advice suggesting the two conflict, is fine to use alongside vitamin C in modern, stabilised formulas.
The common mistake: judging it too early, or expecting it to replace sunscreen
Brightening and tone-related benefits from vitamin C build gradually, generally over eight to twelve weeks of consistent use, not days. Judging a serum before that point is the most common reason people give up on an active that might otherwise have worked. The other frequent mistake is treating a vitamin C serum as sun protection in its own right — its antioxidant role supports skin against everyday environmental stress, but it is not a substitute for a properly applied sunscreen.
Vitamin C serums to consider
Real face-serum listings on Amazon UK tagged with vitamin C in this dataset — see the full face serums page for the complete, sortable list including other actives.
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Frequently asked questions
- Why has my vitamin C serum turned orange?
- That colour change is oxidation — the vitamin C, particularly as L-ascorbic acid, has broken down after exposure to air and light. It is not harmful to use, but it has lost a meaningful share of its antioxidant activity and is worth replacing rather than relying on for results.
- Can I use vitamin C every day?
- Most formulas are suited to daily use, generally in the morning. If your skin is sensitive or new to the ingredient, starting a few times a week and building up gradually reduces the chance of stinging, particularly with higher-concentration L-ascorbic acid formulas.
- Can I use vitamin C with retinol?
- Many people alternate rather than layer them — vitamin C in the morning, retinol at night — since combining two potent actives in one application increases the chance of irritation. Used at separate times of day, both can sit in the same routine without issue.
- How should I store a vitamin C serum?
- Keep it away from direct sunlight and heat, ideally in its original opaque packaging, and use a pump or dropper rather than dipping fingers in, which introduces air and speeds oxidation. A serum that has visibly darkened has already lost potency regardless of how it was stored afterwards.
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Petal & Proof Editorial Team
We do not test products hands-on — see our editorial policy for how listings are sourced and ranked.
Last updated 9 August 2026 · 765 words