Vitamin C Serum: Why Strength Actually Matters for Your Skin

You have probably seen vitamin C serum on every skincare shelf from the pharmacy to the supermarket. Prices range from £8 to £80. The bottles all make similar promises. So what is actually different between them — and does it matter?

The short answer is yes. Significantly.

The concentration problem

Vitamin C — in its most effective form, L-ascorbic acid — is one of the most well-researched skincare actives in existence. The evidence for its benefits is robust: it brightens skin tone, stimulates collagen production, neutralises free radical damage from UV and pollution, and fades hyperpigmentation over time.

But all of that evidence is based on clinically effective concentrations. And most high-street vitamin C serums do not contain them.

A product containing 3% or 5% vitamin C will have minimal impact on most skin types. The clinical research that demonstrates vitamin C's skin benefits typically uses concentrations of 10% to 20%. Below that threshold, you are largely paying for the marketing rather than the ingredient.

Why brands use low concentrations

There are two reasons vitamin C serums are often formulated at low concentrations. The first is cost — L-ascorbic acid at high concentrations is expensive to source and formulate correctly. The second is stability — vitamin C is notoriously unstable and oxidises quickly when exposed to light and air, which is why poorly formulated serums turn orange in the bottle.

The solution to both problems is either to use a low concentration, or to use a derivative of vitamin C that is more stable but less effective than the real thing.

What 20% actually does

At 20% concentration, vitamin C delivers measurable results. Clinical studies at this concentration show significant improvement in skin brightness, reduction in hyperpigmentation, and improvement in skin firmness over consistent use.

The key is formulation. High-strength vitamin C needs to be formulated at the correct pH (between 2.5 and 3.5) to remain stable and effective, packaged to minimise oxidation, and combined with complementary ingredients that support its activity.

C20 by Emerald Derma

C20 is a high strength vitamin C serum formulated at 20% L-ascorbic acid — the concentration at which the clinical evidence for vitamin C's benefits is strongest. It is formulated at the correct pH for stability and efficacy, packaged to protect against oxidation, and designed as part of a complete morning routine.

It is the kind of formulation that, until recently, you would only find through a dermatologist or aesthetic clinic — and at a significantly higher price point.

How to use C20: Apply 3-4 drops to clean, dry skin each morning before moisturiser and SPF. Allow to absorb fully before the next step. Start with alternate days if you have sensitive skin and build up to daily use.

Shop C20 High Strength Vitamin C Serum — £36