# Medik8 C-Tetra

> Medik8 C-Tetra is a lightweight, water-free vitamin C oil with 7% tetrahexyldecyl ascorbate and vitamin E, directed for morning use before sunscreen. It cost £45 for 30ml, or £15 for an 8ml travel size, on Medik8 UK on 25 September 2026. The formula contains grapefruit peel oil.

- Canonical: https://thecastindex.com/beauty/medik8-c-tetra
- Category: Skin prep
- Format: Vitamin C oil serum
- Finish: Lightweight oil
- Best for: A water-free morning vitamin C step
- Official: https://www.medik8.com/products/c-tetra
- Reviewed: 2026-09-25
- Method: evidence-led desk review of official specifications — not an implied hands-on test

## Should you buy this?

A water-free vitamin C oil with 7% tetrahexyldecyl ascorbate and vitamin E, used in the morning before sunscreen, at £45. The derivative is stable but less studied than L-ascorbic acid, and grapefruit oil adds fragrance allergens.

Skip this if you want pure L-ascorbic acid, avoid fragrance allergens such as limonene and citral, or dislike oil textures in the morning.

A water-free vitamin C layer for morning prep — a fine oil to let settle before sunscreen and base.

## The formula

C-Tetra uses tetrahexyldecyl ascorbate, an oil-soluble derivative of vitamin C, at 7% in a lightweight, water-free oil with vitamin E. Derivatives like this are more stable than L-ascorbic acid in the bottle, but there is less published evidence on how much becomes active vitamin C in skin.

Medik8 sells stronger siblings — C-Tetra Luxe at 14% and C-Tetra Advanced at 20% — built on the same derivative.

## Who it’s for

Readers who prefer an oil texture in the morning, or who have found low-pH L-ascorbic acid serums uncomfortable, and anyone wanting a mid-priced first vitamin C with a travel size to try first.

## What should you check before buying?

Despite the “no artificial fragrance” wording, grapefruit peel oil adds fragrance allergens — relevant for fragrance-sensitive skin. The 7% derivative is not directly comparable with 15% or 23% L-ascorbic acid. Let the oil settle before sunscreen so the layers do not slide.

## How to use it

1. Patch test 24 hours before first use
2. Apply four drops in the morning after cleansing
3. Allow to settle, then apply sunscreen

## Questions and answers

### Is Medik8 C-Tetra fragrance-free?

Not strictly. Medik8 says there is no artificial fragrance, but the ingredient list includes grapefruit peel oil and fragrance allergens such as limonene and citral.

### What form of vitamin C is in Medik8 C-Tetra?

Tetrahexyldecyl ascorbate at 7%, an oil-soluble vitamin C derivative, with vitamin E.

### Should I use Medik8 C-Tetra in the morning?

Yes. Medik8 directs four drops in the morning after cleansing, followed by sunscreen.

## Explore the category

https://thecastindex.com/beauty/category/skin-prep