COSRX Vitamin C 23 Serum vs. Dr. Althea Gentle Vitamin C Serum
One is 23% pure ascorbic acid built for visible results fast. The other is a stabilized, beginner-friendly form built not to sting or oxidize. Here's which one actually fits your skin.
Criteria | COSRX The Vitamin C 23 Serum | Dr. Althea Gentle Vitamin C Serum |
|---|---|---|
| Price | $25.00 | $23.00 |
| Price / unit | $1.25/mL | $0.77/mL |
| Size | 20 mL | 30 mL |
| Price checked | 9/12/2026 | 9/12/2026 |
COSRX Vitamin C 23 and Dr. Althea Gentle Vitamin C Serum sit at opposite ends of the same category: one is built for strength, the other for tolerability. The right one depends less on which is "better" and more on how your skin actually handles vitamin C.
Price per milliliter
COSRX runs $1.25 per milliliter ($25.00 for 20mL). Dr. Althea runs $0.77 per milliliter ($23.00 for 30mL). The gap narrows once you account for what's actually in each formula, not just the concentration number on the label.
Concentration and form are the real difference
COSRX uses 23% pure L-ascorbic acid, the most potent and most studied form of vitamin C, positioned for stubborn hyperpigmentation and visible brightening. Dr. Althea uses 3-O-Ethyl Ascorbic Acid, a stabilized derivative, at a lower effective strength, paired with 5% niacinamide and 20% sea buckthorn fruit extract. It's explicitly marketed for first-time vitamin C users and sensitive or acne-prone skin, not as a diluted version of a "real" vitamin C serum.
Stability is a structural difference, not just a strength difference
Pure L-ascorbic acid, the form COSRX uses, is well known for oxidizing quickly once opened, turning yellow or brown as it degrades, which is why potency and freshness matter more with this type of formula. 3-O-Ethyl Ascorbic Acid, the derivative Dr. Althea uses, is inherently more stable and resists that oxidation, a genuine formulation trade-off rather than simply a weaker version of the same ingredient.
Our take
If this is your first vitamin C serum, or your skin reacts easily to new actives, Dr. Althea is the safer starting point, and its stability means less pressure to use it up quickly. If you've used vitamin C before without issues and want the most potent, best-studied form for visible brightening, COSRX is worth the higher price per mL.
For more on how concentration and layering actually work, see how to choose a Korean serum, or browse our full serum buying guide.
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Price checked 9/12/2026
Price checked 9/12/2026
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Frequently asked
Which vitamin C serum is better for beginners, COSRX or Dr. Althea?
Dr. Althea Gentle Vitamin C Serum is the gentler entry point, using a stabilized vitamin C derivative alongside niacinamide and sea buckthorn extract. COSRX's 23% pure ascorbic acid is more potent but also more likely to cause irritation for first-time users.
Why is COSRX Vitamin C 23 more expensive per milliliter than Dr. Althea?
COSRX runs $1.25 per mL versus Dr. Althea's $0.77 per mL, but the gap mostly reflects concentration and ingredient form: pure L-ascorbic acid at 23% costs more to formulate stably than a lower-strength, pre-stabilized derivative.
Does a higher vitamin C percentage always mean better results?
Not automatically. Higher concentrations like COSRX's 23% generally mean faster, more visible brightening, but also a higher chance of irritation and faster oxidation. A lower-strength, stabilized formula like Dr. Althea's can be the more sustainable choice for sensitive skin.