Vitamin C Serum: Benefits, How to Use It, and What to Watch For

Vitamin C (L-Ascorbic Acid) is an important antioxidant for the skin. It stimulates collagen synthesis, helps reduce dark spots and dull skin, and lessens damage from UV light, which makes it one of the most popular serums for brighter skin. In this article, a dermatologist explains the benefits, the correct way to use it, and what to watch out for.

Benefits of Vitamin C on the Skin

• Fades dark spots and evens skin tone: Helps reduce sun-induced dark spots and post-inflammatory hyperpigmentation (PIH), giving a more even skin tone.

• Stimulates collagen: Stimulates collagen production, so skin looks firmer and tighter.

• Boosts UV protection: When used together with sunscreen, it helps strengthen the skin's protection against damage from UV radiation.

• Mild anti-inflammatory effect: Has a mild anti-inflammatory effect.

L-Ascorbic Acid at a concentration of 10-20% is the most effective form, but it must be in a formula with a pH below 3.5 to work well. At this high concentration, it may irritate some people's skin.

How to Use It Correctly

• Use it in the morning: It is best used mainly in the morning, because vitamin C helps boost the effectiveness of sunscreen.

• Follow with sunscreen: Apply vitamin C first, then follow with sunscreen every day.

• Store it properly: Keep it in an opaque container (dark glass bottle) in a cool place away from light, to prevent oxidation from light and oxygen. If the serum turns dark yellow or brown, it has deteriorated.

More Stable Alternatives to L-Ascorbic Acid

If you have sensitive or easily irritated skin, you can choose vitamin C derivatives that are more stable and gentler than L-Ascorbic Acid: Sodium Ascorbyl Phosphate (SAP), Ascorbyl Glucoside (AA2G) and Magnesium Ascorbyl Phosphate (MAP), which are more stable and less irritating.

Frequently Asked Questions

Should I use vitamin C serum in the morning or at night? It is recommended mainly in the morning, because vitamin C helps boost sunscreen's ability to protect the skin from UV, and it should be followed by sunscreen every day.

What concentration of vitamin C works well? L-Ascorbic Acid at 10-20% is the most effective, but it must be in a formula with a pH below 3.5 and may irritate some people's skin. If your skin is sensitive, choose a gentler derivative.

Can people with sensitive skin use vitamin C? Yes, but choose a stable, less irritating derivative such as SAP, AA2G or MAP instead of high-concentration L-Ascorbic Acid.

Why does vitamin C serum change color? Vitamin C oxidizes when exposed to light and oxygen. If it turns dark yellow or brown, it has deteriorated. Store it in an opaque container in a cool place.

If you have dark spots or dull skin, or want to choose a vitamin C that suits your skin, consult a dermatologist at Siam Dermatology. Call +66 61 448 7000 or LINE @siamderm

Written by Dr. Patara Voravutinon, Dermatologist · Last reviewed July 16, 2026