Home / All products / Serums & essences / COSRX
Product photo: cosrx.com (official brand store). The pack size shown may differ from the Korean 20g.
The Vitamin C 23 Serum
A strong pure vitamin C serum with 23% ascorbic acid in a two-phase formula you shake before use and keep in the fridge.
Buyer verdictA real high-strength L-ascorbic acid serum at a low price, cheaper in Seoul than in the US or UK. It isn't for sensitive skin, and it needs the fridge.
- Key ingredient
- Ascorbic acid (23%), 3-O-ethyl ascorbic acid
- Texture
- Two-phase liquid that you shake; slightly slippery, can tingle
- Routine step
- serum · AM
- Korean pack
- 20g
Overview
Key ingredients
- Ascorbic acid (23%) Pure vitamin C, the most studied form for brightening and antioxidant protection
- 3-O-ethyl ascorbic acid A more stable vitamin C derivative
- Panthenol and squalane Soothe and soften to offset the strength
- Glutathione, niacinamide and tocopherol Small supporting antioxidants and tone ingredients
From the US, it's 44% cheaper in Seoul after tax and refund. From the UK, it's 23% cheaper in Seoul.
What it is
COSRX’s The Vitamin C 23 Serum, now sold at Olive Young as the Advanced version, is a 20g serum built around 23% ascorbic acid. That’s pure L-ascorbic acid, the original form of vitamin C, at a strength most brands don’t go above.
The Korean ingredient list starts with water, ascorbic acid (23%), butylene glycol and dimethicone, then panthenol, 3-O-ethyl ascorbic acid and squalane. Smaller amounts of caffeine, sodium hyaluronate, adenosine, niacinamide, glutathione, tocopherol and plant oils follow. It’s a two-phase formula, oil and water, so you shake it before each use.
Who it suits (and who should skip it)
Good pick if
- You want a genuinely strong vitamin C for dull skin and dark marks
- Your skin already tolerates acids or actives
- You’ll use a 20g bottle up within a few months
Skip it if
- Your skin is sensitive, reactive or easily stings
- You don’t want to keep skincare in the fridge
- You’re new to vitamin C; start with a gentler one
What the ingredients do
Ascorbic acid is the most researched form of vitamin C on the skin. It’s an antioxidant that helps against daily damage from UV and pollution, and with regular use it can fade dark marks and brighten dull skin. It’s also unstable and acidic, which is why high-strength versions can sting and need careful storage.
3-O-ethyl ascorbic acid is a more stable derivative, which adds some vitamin C activity that lasts longer in the bottle. Panthenol and squalane help keep skin comfortable. The small amounts of glutathione, niacinamide and tocopherol play supporting roles; tocopherol (vitamin E) is a classic partner for vitamin C.
How to use it
COSRX’s directions: attach the dropper, then shake the bottle once or twice before use. Don’t shake it before the dropper is fitted. After cleansing, drop a suitable amount onto the face at the serum step and roll it in with your fingertips from the centre outwards. Use sunscreen if you apply it in the daytime.
Start every other morning, and leave it out on nights you use retinoids or strong acids. Our serum buying guide covers how to layer actives.
How it compares
Klairs Freshly Juiced Vitamin Drop is the gentle, low-strength option for sensitive skin. Goodal Green Tangerine Vita C Serum is a mild everyday brightening serum. This COSRX is the strongest of the three.
Is it worth buying in Seoul?
Yes. Seoul was cheaper than both Amazon US and Amazon UK in our latest check, and one bottle clears the ₩15,000 tax refund minimum. Buy it near the end of your trip if you can, and keep it out of the heat on the way home, because pure vitamin C doesn’t like warmth. At 20g it’s fine for carry-on.
FAQ
Does it tingle?
It can. 23% ascorbic acid is a high strength and pure vitamin C serums are acidic. Start every other day.
Why keep it in the fridge?
COSRX says to store it upright in the fridge (3-10°C) and start using it as soon as you buy it. Pure vitamin C breaks down faster in warmth and light.
Is it a functional cosmetic?
Yes. The Korean label says it passed Korea's functional cosmetic review, which covers brightening claims.
Is it the same as the older version?
Olive Young now lists it as Advanced The Vitamin C 23 Serum. Check the name on the box against what you're comparing.
