COSRX The Vitamin C 23 Serum
A high-strength pure L-ascorbic acid serum in a thick, slightly syrupy base, and the strongest rung of COSRX's vitamin C ladder — the same line runs a gentler low-percentage version for people who cannot take this one. Pure ascorbic acid is the best-evidenced form of topical vitamin C for the look of tone and brightness, but it is also the most unstable and the most likely to sting. Nothing about this formula hides that.
What it is built around
Who it suits
For normal, oily or combination skin that already tolerates actives and wants the strongest well-documented form of vitamin C rather than a comfortable derivative. Good for the look of uneven tone, dullness and lingering post-blemish marks. Sensitive, rosacea-prone or barrier-compromised skin should not start here — the lower-strength version of the same serum exists for exactly that reason.
How to use it
Morning, on clean dry skin, before anything else — a few drops is plenty and more is genuinely wasteful. Wait a minute, then moisturiser and sunscreen. Sunscreen is not optional with this one; vitamin C is a poor substitute for it and a good partner to it. Start two or three mornings a week and build up only if your skin stays calm.
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🙋 People also ask
What is the difference between essence and serum?
In Korean skincare, essence is typically a thinner, hydration-focused layer that preps skin, while serum is more concentrated with targeted actives like vitamin C or niacinamide. Essence usually comes first; serum follows. The line between them blurs, and not every routine needs both.
Can you use vitamin C and niacinamide together?
Yes, current understanding is that vitamin C and niacinamide can be used together safely, and many Korean formulas combine them. The old warning that they cancel out is largely outdated. If your skin is sensitive, you can still apply them at different times of day to be cautious.
Can I use vitamin C and retinol together?
Not in the same layer, but easily in the same routine — vitamin C in the morning, retinol at night. That split suits both: vitamin C partners with sunscreen, and retinoids are degraded by light.
What is the difference between essence, serum and ampoule?
Essence is the lightest and most hydration-focused, serum is more concentrated and targeted, and ampoule is the most concentrated of the three. The boundaries are marketing convention, not regulated definitions.
✍️ Written & reviewed by the KoreaPlus Editorial team — dermatologist-informed, cosmetic-science researched & source-cited. Last reviewed: 2026-08-23.
General educational information using cosmetic structure-function wording — not medical advice. Always patch-test new actives. © KoreaPlus.