# Vitamin C

> Source: https://thecastindex.com/beauty/ingredients/vitamin-c — updated 2026-09-25

Vitamin C is an antioxidant used in serums to help skin look brighter and more even, and to support protection against environmental damage alongside sunscreen. Pure L-ascorbic acid is the most studied form but the least stable; derivatives are gentler and more stable, though evidence for them is more limited.

- Also on labels as: L-ascorbic acid, Ascorbic acid, Tetrahexyldecyl ascorbate, 3-O-ethyl ascorbic acid, Ascorbyl glucoside
- Type: Antioxidant

## What it does

- Antioxidant support that complements, but never replaces, sunscreen
- Helps the look of dullness and uneven tone
- Plays a role in collagen production in skin

## Who it suits

Most skin types looking for brightness. Sensitive skin may prefer a derivative or a lower-strength formula.

## How to use it

1. Usually applied in the morning after cleansing, before moisturiser and sunscreen
2. Store as directed — many formulas degrade with air, light and heat
3. Discard if a pure L-ascorbic acid serum turns dark orange or brown

## Take care

- High strengths of L-ascorbic acid can tingle or sting
- Patch test if you have sensitive skin

## What it pairs with

Works well with vitamin E and ferulic acid, and before sunscreen. Keep strong exfoliating acids and retinoids for the evening unless a product says otherwise.

## Products in the catalogue that list it

- [SkinCeuticals C E Ferulic](https://thecastindex.com/beauty/skinceuticals-c-e-ferulic)
- [Medik8 C-Tetra](https://thecastindex.com/beauty/medik8-c-tetra)
- [The Ordinary Vitamin C Suspension 23% + HA Spheres 2%](https://thecastindex.com/beauty/the-ordinary-vitamin-c-suspension-23-ha-spheres-2)
- [Beauty of Joseon Light On Serum: Centella + Vita C](https://thecastindex.com/beauty/beauty-of-joseon-light-on-serum-centella-vita-c)

## Questions

### Should vitamin C be used morning or night?

Most people use it in the morning, under sunscreen, for its antioxidant role. Follow the directions on your specific product.

### What is the most effective form of vitamin C?

L-ascorbic acid has the most research behind it but is unstable and can irritate. Derivatives such as tetrahexyldecyl ascorbate and 3-O-ethyl ascorbic acid are more stable and usually gentler.

### Why did my vitamin C serum turn orange?

L-ascorbic acid oxidises when exposed to air, light and heat, which darkens the serum. A strongly darkened serum is past its best.

## Sources

- [Pullar et al., “The Roles of Vitamin C in Skin Health”, Nutrients, 2017](https://pmc.ncbi.nlm.nih.gov/articles/PMC5579659/)

General information, not medical advice.