A high-strength vitamin C serum built on 23% pure L-ascorbic acid — the highest concentration COSRX produce, and among the most potent available without a prescription. This is a treatment product for experienced users, not an entry point.

L-ascorbic acid is the only form of vitamin C the skin can use directly, without conversion. It is also the most difficult to formulate: it demands a low pH to penetrate, and it oxidises rapidly on exposure to light, heat and air. Derivatives are gentler and more stable but weaker. This serum takes the harder route and delivers the real thing at full strength.

What vitamin C actually does: it is a potent antioxidant, neutralising the free radicals generated by UV exposure and pollution that degrade collagen over time. It inhibits tyrosinase, so it works on dark spots and uneven tone. And it is a required cofactor in the skin's own collagen synthesis. Used every morning under sunscreen, it is one of the best-evidenced anti-ageing steps available.

Understanding 23%

Research generally identifies 8% as the threshold for measurable effect and around 20% as the point beyond which absorption plateaus. At 23% this sits at the top of the useful range — powerful, and correspondingly more likely to sting or irritate. If you have never used vitamin C, start lower. COSRX make a 13% version, and there are gentler derivative-based options in our range. If you already tolerate vitamin C comfortably, this is the step up.

Key ingredients

• Ascorbic acid (23%) — pure L-ascorbic acid, second on the ingredient list. Antioxidant protection, brightening, and support for the skin's own collagen production.
• Ethyl ascorbyl ether — a stable, longer-acting vitamin C derivative running alongside the pure form, so activity continues after the ascorbic acid has done its work.
• Glutathione — a powerful antioxidant widely used in Asia for brightening, and one that regenerates vitamin C after it has neutralised a free radical.
• Niacinamide — supports the barrier and works on tone. (The old advice that niacinamide and vitamin C cannot be combined has been largely set aside; formulating them together is now common and well tolerated.)
• Tocopherol and tocotrienols (vitamin E) — vitamin C and vitamin E are markedly more effective together than separately, each regenerating the other.
• Caffeine — antioxidant support with a mild depuffing effect.
• Acetyl glucosamine — works on pigment transfer and supports hyaluronic acid production in the skin.
• Licorice root extract — a gentle brightener that also calms.
• Panthenol, allantoin and squalane — soothing and emollient support, offsetting the potential harshness of a low-pH formula.
• Sodium hyaluronate and adenosine — hydration and fine-line support.

How to use

1. Use in the morning. Vitamin C's antioxidant work happens during daytime UV and pollution exposure, so morning is when it earns its place.
2. Cleanse and tone, then let the skin dry completely. Applying a low-pH vitamin C to damp skin increases stinging significantly.
3. Apply 3 to 4 drops to the face. More is not better.
4. Pat gently and allow 60 to 90 seconds to absorb before the next step.
5. Follow with moisturiser, then SPF 50 — mandatory.

Introducing it

Start with every third morning for the first week, every other morning in week two, then daily if comfortable. A brief tingle on application is normal at this concentration. Sustained burning, stinging or redness is not — reduce frequency or buffer over moisturiser.

Recommended frequency

• Experienced vitamin C users: once daily, every morning.
• New to high-strength vitamin C: build up over two to three weeks as above.
• Sensitive skin: every other morning, applied over a layer of moisturiser to buffer.
• Do not use in the same routine as AHAs, BHAs, benzoyl peroxide or a retinoid. Vitamin C in the morning, those actives at night, is the correct arrangement.
• Timeline: brightness and glow at 2 to 4 weeks. Visible fading of dark spots at 8 to 12 weeks. Antioxidant protection is working from day one, even though you cannot see it.

Best suited to

Dullness, uneven tone, sun damage, dark spots and early signs of ageing. Experienced actives users. Anyone who has been using a lower-strength vitamin C and wants more. Not recommended as a first vitamin C, for very sensitive or rosacea-prone skin, or for anyone with a compromised barrier.

Specifications

• Size: 20 g
• Product type: Leave-on morning treatment serum
• Key active: Ascorbic acid (pure L-ascorbic acid) 23%
• Strength: High — advanced users
• Supporting actives: Ethyl ascorbyl ether, glutathione, niacinamide, vitamin E, caffeine, acetyl glucosamine, licorice root, panthenol, squalane
• Texture: Lightweight fluid serum
• Use: Morning
• Dosage: 3–4 drops
• Country of origin: Republic of Korea
• Approximate usage: 6–8 weeks at daily use

Good to know — storage matters more than usual

Pure L-ascorbic acid oxidises. As it does, the serum turns from clear or pale yellow to deep yellow, then orange, then brown. A brown serum is an inactive serum and should be replaced — a light yellow tint is normal and fine. To slow this down: keep the cap tightly closed, store away from light and heat, and consider keeping it in the fridge. Use within three months of opening.

The formula also contains a small amount of denatured alcohol, which aids penetration but may not suit very dry skin. Daily SPF 50 is required — vitamin C increases photosensitivity, and using an antioxidant without sun protection wastes both. Avoid the eye area. Do not apply to broken or actively inflamed skin. Patch test on the jawline for 24 hours before first use. Discontinue if persistent burning develops. Keep out of reach of children.