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This is a review of the new Medik8 C-Tetra Advanced Gel Serum — but it’s not just about this. It’s about why ‘20% vitamin C’ on a label should make you take a closer look, so you know exactly what you are getting before you purchase. Especially at 85 €.

Because 20% tetrahexyldecyl ascorbate is not the same thing as 20% ascorbic acid. And if you have sensitive or aging skin and you’ve ran into trouble with every Vitamin C you’ve tried so far, this could make all the difference.

So I’ll show you how this serum behaves on the skin, where it fits into a realistic morning routine, and how it compares to C-Tetra Luxe — so you can decide whether this actually makes sense for you.

What the Medik8 C-Tetra Advanced claims

Defend against premature skin ageing and achieve accelerated skin brightening with a next-generation combination of 20% vitamin C and skin barrier science.

Clinically proven to brighten skin and visibly improve wrinkles in 7 days with 0% barrier disruption, C-Tetra Advanced is engineered for high-performance visible brightening, skin smoothing, and daily antioxidant defence.

This intelligent ingredient complex combines potent, stabilised 20% vitamin C with restorative phyto exosomes for enhanced skin resilience and omni-antioxidant acetyl zingerone to neutralise the ageing effects of free radicals.

The result? A high-performance, age-defying vitamin C serum that works smarter, not harsher.

Facts about the Medik8 C-Tetra Advanced 20% Vitamin C Gel Serum

Prize and size

The serum comes in a dark brown glass bottle with a pump that contains 30 ml and is available for 85 € on the website here. The pump works very well. One pump is enough product for a small face, you’ll need two if you want to do face and neck, and probably three if you take it down on the decollete (which you should, read here why).

Texture and smell

I expected something clear and light given it is called a gel serum, but it looks and feels more like a lightweight lotion and is milky-white. The scent is absolutely delicious, it reminds me of lemon meringue, and it spreads with ease and leaves no stickiness behind.

How to use the Medik8 C-Tetra Advanced 20% Vitamin C Gel Serum

There is a point to be made for using your Vitamin C products in the mornings – the antioxidant effect helps catch the free radicals that UV exposure produces during the day – as well as using them at night – the product isn’t „used up“ for free radicals, so it can be of more support in the collagen production side of things. In the end it comes down to what you prefer and where it fits your routine best. I use my Vitamin C in the mornings, mainly because I want to keep my routines as short as possible and I already have quite a bit going on at night.

If you are using L-Ascorbic Acid, that gets even more important as that one is quite unstable and often formulated in a way that might irritate your skin, so it is important with what you pair it with. In this case, the product uses Tetrahexyldecyl Ascorbate, a Vtamin C derivative, meaning it is more stable and has less irritation potential, which means it matters less what else you use alongside it.

Medik8 recommends using it twice daily, which you absolutely could. With using two pumps once a day though I reckon I will finish the bottle within three months, so using it twice a day could be a relevant cost factor.

No matter which sunscreen or other serums I paired it with, there was no weird reactions, no pilling or sliding around, it went well with everything. The slightly more lotiony texture means you would probably be able to use just this one without a moisturizer in the warmer months if your skin leans more oily. It is quite hydrating on its own, meaning it is a great option for those that want to keep their routine as minimal as possible.

How do 20% Tetrahexyldecyl Ascorbate and 20% L-Ascorbic Acid compare?

Before we dive into the general ingredient breakdown, let’s look at the Vitamin C derivative used here: Tetrahexyldecyl Ascorbate. Let’s start it off with looking at L-Ascorbic Acid, the one we have the most data on and the one that has been around the longest. It is quite important for skin health for several reasons:

• Potent antioxidant: Over the course of the day, different things lead to the production of free radicals in our skin that lead to damage. Antioxidants have the ability to neutralize these free radicals and protect the skin, preventing premature skin aging.
• Hyperpigmentation reduction: Vitamin C hinders a step in the melanin production pathway, so having it present means hyperpigmentation you have in your skin will be reduced over time, leading to more even and brighter looking skin (only if you wear your sunscreen of course!)
• Collagen production: Vitamin C is a cofactor in the production of collagen, so having it present helps maintain and rebuild collagen, leading to firmer looking skin and eventually a reduction in wrinkles

Sadly, it also has a few disadvantages limiting its use in skincare, the first one being that it is very unstable – that is the nature of the antioxidant. This means if not formulated correctly, it has most likely already degraded in the bottle before you even opened it. To prevent that, it is often combined with other ingredients and formulated with a quite low pH (which is also supposed to help with penetration in the skin, another thing it doesn’t do too well). That low pH is a source of irritation for some people, especially for those with rosacea, limiting its use there.

To counteract that, derivatives were made, most of them with a higher stability and a better penetration potential into the skin. The goal was to get them from the bottle into the skin where they’d be transformed into L-Ascorbic Acid and worked as described above, just without all the hurdles beforehand. Tetrahexyldecly Ascorbate is one of the oil soluble derivatives with potentially easier skin penetration, and is also more stable, so it doesn’t need a low pH.

But while that sounds promising and might work, we don’t really have nearly as much data on them as we have on L-Ascorbic Acid, and quite a lot of the data we have is manufacturer studies. Basically: there is potential and some studies have shown promising results, but there is still a lot we don’t know enough about to say for sure.

As for the concentration: 15-20% L-Ascorbic Acid is the recommended concentration. There are lower concentration available that come with a lower irritation potential but probably longer time until you see results, and higher concentrations which won’t give you added benefits, only added irritation potential. For Tetrahexyldecly Ascorbate, there is no ideal concentration we know of. It is used in concentrazions between 5 up to 30%, but which one is superior isn’t something I can say. As up to 30% apparently don’t increase irritation, you can probably use it just fine, but most products available won’t give you more than 20%.

Now which one is better for you? That entirely depends on you and your skin. I personally have found a 15% Ascorbic Acid that is affordable, stable, never irritating and has given me results (it is this one), but if you find regular Vitamin C irritating or want to be on the safe side because your skin has had irritation in the past, trying a derivative might be a good option. This one though wouldn’t be my first choice for reactive skin, and I will tell you why when we discuss the ingredients.

Ingredients of the Medik8 C-Tetra Advanced 20% Vitamin C Gel Serum

Water solv, Tetrahexyldecyl Ascorbate aox|sb, Isododecane emo|solv, Propanediol solv|h, Acetyl Glucosamine sii|sb, Squalane sii|emo 0 1, Tocopherol aox 0–3 0–3, Acetyl Zingerone aox|chel, Camellia Sinensis Leaf Vesicles (Green Tea), Hydroxyacetophenone aox|pres, Sodium Hyaluronate sii|h 0 0, Cellulose vc, Avena Sativa (Oat) Kernel Extract aox|so|emo|abrasive/scrub, Glycerin sii|h 0 0, Hydroxyethyl Acrylate/Sodium Acryloyldimethyl Taurate Copolymer vc, Citrus Grandis (Grapefruit) Peel Oil perf, Sucrose Stearate emo|emu|surf 0 0, Tetrasodium Glutamate Diacetate chel, Ethylhexylglycerin pres, Acrylates/C10-30 Alkyl Acrylate Crosspolymer vc, Xanthan Gum vc, Sodium Benzoate pres, Cellulose Gum vc 0 0, 1,2-Hexanediol solv, Polysorbate 60 emu|surf, Sorbitan Isostearate emu 0 1–2, Citric Acid buff, Phenoxyethanol pres, Citral perf, Linalool perf, Limonene perf|solv

Hover the mouse over an ingredient for short explanation. Read more on INCIDecoder.
Other than the 20% Tetrahexyldecly Ascorbate we already discussed, the serum contains Grapefruit Peel Oil. Now I am not against fragrant skincare or essential oils, I love a good scent as part of relaxing at night, but especially essential oils are often a trigger for those whose skin reacts easily. Again, not a problem for the majority of people, but if you know your skin is particularly reactive, I’d advise patch testing first. (More info: Is fragrance in your skincare dangerous for you?)

Now what about the skin soothing claims? I assume the Camellia Sinensis (Green Tea) Leaf Vesicles are what they mean when they mention phyto exosomes, but I can’t be entirely sure. While exosomes are quite popular in skincare right now (I’ve talked more about them here), these particular ones aren’t ones I have come across often, so I can’t comment on their effect. The Avena Sativa Kernel (Oat Meal) Extract on the other hand contains Beta Glucan, which is known for its skin soothing properties. (More info: The benefits of colloidal oat meal in skincare explained).

The great hydration is most likely due to the Glycerin, the Sodium Hyaluronate and the N-Acetyl-Glucosamine present, the latter being one of the Natural Moisturizing Factors your skin produces on its own. (More info: What are Natural Moisturizing Factors?). Squalane as emollient is also present and explains why I like the texture so much, and lastly you’ll find Acetyl Zingerone, an antioxidant derived from ginger.

More info: How to read the ingredient list in your skincare products

Does it live up to its claims?

I can’t say for sure.

It is a beautiful product, but to live up to its claims, you need to believe in the effects of Tetrahexyldecyl Ascorbate. It is a promising derivative, but the data isn’t there in the same way it is for Ascorbic Acid. As I have been using a Vitamin C Serum in some shape or form for the better part of the last 20 years, this one didn’t give me a dramatic effect.

How does it compare to…

… the other Medik8 C-Tetra serums?

I haven’t tried the regular C-Tetra one, which is available for 56 €, has a lightweight oil texture and contains 7% Tetrahexyldecyl Ascorbate, but I did try the C-Tetra Luxe one. That one is 82 €, quite a rich oil in texture and has the same, beautiful scent. It has 14% Tetrahexyldecyl Ascorbate, so sits right in the middle of the three. For me personally, the lotiony texture of the C-Tetra Advanced one was much more agreeable, but that is personal preference. As both only differ by three € in price and there is no way to tell if 14% is better or worse in terms of results than 20%, I’d suggest going with the one you prefer texture-wise.

You can read my full review of the Medik8 C-Tetra Luxe Serum here.

Geek & Gorgeous C-Glow Serum

I have talked about this one so often on here, this might sound like an advertising platform, but I am just genuinely excited about it. It feels like nothing on my skin, never oxidised on me, isn’t irritating (again – for me and my skin) and is available for a low price at my local drugstore. The scent isn’t as delicious as the Medik8 one and it isn’t hydrating, so depending on the season you might need a separate hydration step, but it still is the one I keep going back to.

You can read my full review of the Geek and Gorgeous C-Glow Serum here.

Sunday Riley CEO 15% Vitamin C Brightening Serum

Another one with Tetrahexyldecyl Ascorbate, this time 15%, and sadly another one that pairs it with fragrant essential oils. It has a similar texture and also a beautiful scent and is in the same, more luxurious price range. Both make your routine feel really luxurious and are hydrating enough to eliminate the need for a different product to pair it with.

You can read my full review of the Sunday Riley CEO 15% Vitamin C Brightening Serum here.

Would I repurchase and which skin types do I recommend the Medik8 C-Tetra Advanced 20% Vitamin C Gel Serum to?

While I love texture and scent of the Medik8 C-Tetra Advanced Gel Serum I will personally stick to my regular Vitamin C using L-Ascorbic Acid. The main reason is that for Ascorbic Acid, we have a lot more data than we have for the derivatives like Tetrahexyldecyl Ascorbate. If you don’t get along with Ascorbic Acid due to irritation and the low pH it needs, or want something that is more stable so you don’t have to worry about it oxidising, then Medik8 could be a good option for you. Unless you react to fragrant essential oils, then I’d be careful.

Shop Medik8 here

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