Medik8 C-Tetra Review

Medik8 C-Tetra is a 7% Tetrahexyldecyl Ascorbate vitamin C serum built as the brand's entry point for vitamin C beginners and sensitive skin. Its oil-based, stabilised formula sits at the gentle end of Medik8's vitamin C range, sitting below C-Tetra Luxe and Super C Ferulic for anyone easing into an antioxidant morning routine.

What is Medik8 C-Tetra?

C-Tetra is a lightweight, oil-based serum built around 7% Tetrahexyldecyl Ascorbate, a lipid-soluble, stabilised derivative of vitamin C. Unlike traditional L-ascorbic acid, which is water-soluble and prone to oxidising quickly once opened, this form is designed to stay stable in an oil base, so the serum keeps working from the first drop to the last. Medik8 positions it as the gentlest vitamin C serum in its line-up, which is also why it's the product the brand points beginners and sensitive-skin users towards first.

The formula is finished with Vitamin E, added for its own antioxidant properties and to work alongside the vitamin C as a network antioxidant pairing, plus a jojoba seed oil base that carries both actives into the skin without a heavy or greasy after-feel.

Why a lipid-soluble vitamin C instead of ascorbic acid?

Most of the sting, redness and short shelf-life associated with vitamin C serums comes down to L-ascorbic acid, the most common and most researched form of vitamin C — but also the least stable. It's water-soluble, oxidises quickly once a bottle is opened, and can irritate reactive skin at higher strengths. Tetrahexyldecyl Ascorbate takes a different route: it's a fat-soluble ester of vitamin C, which means it sits comfortably in an oil base, resists oxidation for longer, and is generally considered gentler on the skin barrier. The trade-off is that ester forms are typically regarded as less potent, drop for drop, than pure ascorbic acid — part of why C-Tetra is positioned as the gentle starting point rather than the maximum-strength option in the Medik8 line-up.

Who is C-Tetra formulated for?

Medik8 positions C-Tetra specifically for:

• Anyone trying a vitamin C serum for the first time
• Sensitive skin that reacts to stronger acid-based vitamin C formulas
• People wanting to even out the appearance of dull or uneven skin tone
• Those building a morning CSA routine (vitamin C, sunscreen, and vitamin A at night) who want the vitamin C step to be low-risk
• All skin types looking for a daily antioxidant step, from dry through to oily and combination skin

Because the formula is built to be tolerated well, it's also a common starting point before stepping up to a higher-strength option like Super C Ferulic, or a richer, more indulgent formula like C-Tetra Luxe, once skin has adjusted to vitamin C.

Texture, scent and how it wears under SPF

C-Tetra has a silky, oil-based texture rather than the thinner, watery feel of an ascorbic acid serum. It absorbs without sitting greasy on the skin, which means it layers reasonably well underneath a moisturiser or SPF rather than pilling. The scent comes from natural grapefruit peel oil, giving it a light citrus note rather than a heavier fragrance. As with any facial oil-based serum, a small amount goes a long way, and applying too much in one go is more likely to leave a residual sheen.

What results does Medik8 report?

Medik8's own product testing, published on the C-Tetra product page, reports that 96% of sensitive skin participants in the brand's trial found it the gentlest vitamin C serum they had used, and 90% agreed their skin felt softer after use. The brand also states that consistent use over several weeks was associated with a visibly brighter-looking complexion and a reduction in the appearance of fine lines, dark spots and hyperpigmentation in its own testing.

These are the brand's reported findings rather than independently verified clinical data, so treat them as Medik8's own claims. What the formula is designed to do — and what the ingredient list supports — is help improve the appearance of dullness, uneven tone and the visible signs of environmental and oxidative stress, while providing daily antioxidant support alongside SPF.

Packaging and stability

C-Tetra comes in an opaque, air-restricted bottle with a dropper, which matters more for vitamin C than most other actives: light and repeated air exposure are the two biggest causes of a vitamin C serum losing potency after opening. Combining that opaque packaging with an oil-soluble form of vitamin C is part of why Medik8 states the formula stays stable and effective through to the last drop, rather than degrading a few weeks after the seal is broken — a common complaint with clear-bottled ascorbic acid serums.

How to use it

• Apply a few drops to clean, dry skin in the morning, before moisturiser and SPF
• Vitamin C serums like this sit at the start of the CSA routine — vitamin C by day, sunscreen always, vitamin A by night
• Store away from direct sunlight and heat to help protect the stability of the formula
• Patch test first if you're new to vitamin C or have reactive skin, even though the formula is designed to be gentle

If you're pregnant or breastfeeding, please contact our team for advice before use.

C-Tetra vs C-Tetra Luxe vs Super C Ferulic

Medik8 runs several vitamin C serums side by side, and it's easy to end up unsure which one to start with. Here's roughly how C-Tetra sits against the other two:

• C-Tetra — the entry point: 7% Tetrahexyldecyl Ascorbate in a light, oil-based formula. Built for vitamin C beginners and sensitive skin.
• C-Tetra Luxe — a richer, more indulgent version of the same stabilised vitamin C technology, aimed at those who want a more luxurious sensorial step up once skin is used to vitamin C.
• Super C Ferulic — a higher-strength, more advanced vitamin C serum aimed at users who already tolerate vitamin C well and want a more potent antioxidant formula.

For a full side-by-side breakdown of strength, texture and who each one suits, see our C-Tetra vs Super C Ferulic comparison.

Where C-Tetra fits in the wider vitamin C line-up

If you're weighing up more than just these two, our guide to the best Medik8 vitamin C serums covers the full range, including where Daily Radiance Vitamin C — Medik8's vitamin C moisturiser with SPF — fits for anyone who wants their vitamin C and sun protection combined into one step. You can also browse the full Medik8 brand hub for the rest of the range.

Our verdict

C-Tetra does what Medik8 designed it to do: it gives beginners and sensitive skin a genuinely gentle way into vitamin C, without the sting or instability that can put people off traditional ascorbic acid serums. It's not the most potent vitamin C option in the Medik8 range, and it isn't meant to be — that's what Super C Ferulic and C-Tetra Luxe are for. As a first vitamin C serum, or a long-term choice for reactive skin, it's a sensible, low-risk starting point.