Vitamin C serum: which one, when, and for whom

Two questions before buying a vitamin C serum: which form, and what percentage. Get these wrong and it either stings or does nothing.

Form: pure or derivative

• L-ascorbic acid (LAA): the most researched, the fastest results, and the least stable. Air, light and heat turn it orange within weeks; orange means it has stopped working. Can sting on sensitive skin.
• Derivatives (ascorbyl glucoside, ethyl ascorbic acid, SAP, 3-O-ethyl): stable, gentle, slower. For Dhaka heat and for beginners these are the better choice.
• Most Korean serums are derivatives or low-percentage LAA, which is why they feel "light".

Percentage

• Starting out: 5–10%. Sensitive skin: a derivative, under 10%.
• 15–20% LAA: for skin that is used to it; if it stings or reddens, step down. A higher percentage is not more result.

When

In the morning, after cleanser and toner, before sunscreen. The extra defence against sun damage is only useful in the morning. At night is fine too, just not on the same night as retinol or AHA/BHA.

What it goes with, and what it does not

• Goes with: niacinamide (the old "never together" claim does not hold for modern formulas), hyaluronic acid, sunscreen, vitamin E / ferulic acid.
• Not together: retinol (at night, separately), AHA/BHA (the pH clash stings), benzoyl peroxide.

Buy by the bottle

Opaque or dark bottle, pump or dropper; avoid clear glass and wide jars. Finish within three months of opening; the fridge makes it last longer. Orange-brown means throw it away.

Picks

• Starting out, any skin: Medicube Deep Vita C Capsule Serum 30ml, Beauty of Joseon Light On Serum Centella + Vita C 30ml
• Used to it, higher percentage: Cos De BAHA Vitamin C 15% Serum 60ml, Torriden Cellmazing VITA C Brightening Ampoule 30ml
• Serum feels heavy? The cream form: Medicube Deep Vita C Capsule Cream 55g
• On top in the morning: Beauty of Joseon Relief Sun Aqua-Fresh Rice+B5 50ml

What to expect

Glow in two or three weeks; marks lighten over 8–12 weeks. Skin does not turn "fair"; dullness lifts and tone evens out.

Vitamin C in the morning without sunscreen is half the job. The full routine for marks: melasma and dark spots guide.

Sources

• Published dermatology reviews: topical vitamin C at 8–20% is effective for photodamage and hyperpigmentation; LAA is unstable, derivatives are stable but slower.
• The vitamin C + niacinamide "harmful" myth comes from 1960s tests on unstable formulas; in modern formulas the pair is safe.