COSRX The Vitamin C 23 Serum is the high-strength end of the brand's vitamin C range, built around 23% pure ascorbic acid in a 20ml dropper bottle. It belongs at the serum step, going on after your toner and before moisturizer.

The formula separates into two layers on the shelf, so a gentle shake brings them back together before each use. Ascorbic acid does the visible tone work, and tocotrienols — what COSRX calls Super Vitamin E — sit alongside it as a second antioxidant. Sodium hyaluronate and panthenol hold water at the surface, allantoin keeps the finish comfortable, and a light layer of squalane with camellia and safflower seed oils softens the drag a strong acid serum can otherwise leave.

Dull, uneven-looking skin is where it earns its place, and it settles to a dewy finish rather than a matte one. COSRX builds the 13% version for people newer to vitamin C and calls that one the everyday choice for reactive skin, so think of 23% as the step up once a lower strength already sits comfortably.

The bottle is deliberately small. Pure ascorbic acid is reactive, so COSRX suggests refrigerating it once opened and finishing it inside two months. There is no added fragrance, though vitamin C carries a faint scent of its own. Patch test on your wrist or neck before the first full application.