Choosing the Right Vitamin C Serum for Your Skin (and Why the Highest Percentage Isn't Always Best)

When patients come in asking about vitamin C, there's a phrase Dr Yasmin Tayeb, doctor and founder of Wellface, hears almost every week: "I want the 20 percent, the strongest one you've got." It's an understandable instinct. If a serum is good for your skin, surely more of it is better. But that isn't quite how vitamin C works, and reaching for the highest concentration by default can leave sensitive or dry skin feeling tight, flaky and irritated rather than glowing.

The products you put on your skin have a genuine effect on how it looks and behaves, especially when you're using medical-grade formulas. These aren't inexpensive, so it makes sense to get the right one rather than the most powerful one. Vitamin C is one of the best-studied ingredients in skincare, and the concentration that suits you depends on your skin type, how much oil or dryness you're prone to, and even the time of year.

In this video, Dr Yasmin Tayeb walks through what vitamin C actually does, how the 10, 15 and 20 percent strengths differ, and why she often moves patients down a level as the colder seasons arrive. Think of it as the conversation you'd have across the consultation room, written down.

What vitamin C actually does for your skin

Vitamin C, listed on labels as L-ascorbic acid, is the most studied and most reliable form of the ingredient. It's a potent antioxidant, which means it helps neutralise the free radicals your skin encounters every day from UV light and pollution. That protective role matters because free radical damage is one of the drivers of premature ageing.

Beyond protection, vitamin C plays an active part in collagen production. Your skin needs it to build and maintain the collagen that keeps the complexion firm and springy. It also helps to fade pigmentation by slowing the formation of melanin, which is why so many patients notice a more even, brighter skin tone after a few months of consistent use. Dark spots soften, dullness lifts, and the skin simply looks healthier.

Used in the morning under your SPF, vitamin C and sunscreen work well together. The serum mops up some of the free radicals that get through even the best sunscreen, and the SPF shields the skin from the UV that would otherwise undo your progress. One doesn't replace the other. They're a pair.

Why concentration is not the whole story

Most of the clinical evidence points to a sweet spot between roughly 10 and 20 percent L-ascorbic acid. Below about 8 to 10 percent, you don't get the full benefit. Above 20 percent, you don't get meaningfully better results, and you do increase the risk of irritation. So the numbers aren't a simple ladder where the top rung wins. They're a range, and your job is to find the rung your skin sits on comfortably.

Stability matters too. Vitamin C is a temperamental ingredient that degrades when exposed to light, heat and air. That's why good serums come in dark or tinted glass and why the formula sometimes pairs vitamin C with vitamin E and ferulic acid, which help keep it stable and boost its antioxidant effect. If your serum has turned a deep orange or brown, or developed an off smell, it has oxidised and won't do much for your skin. Keep the bottle sealed and stored somewhere cool and dark.

10, 15 or 20 percent: matching the strength to your skin

At Wellface Dr Yasmin Tayeb uses Obagi vitamin C serums, which come in 10, 15 and 20 percent strengths. The three exist for a reason. They let her tailor the dose of active vitamin C to the person in front of her rather than handing everyone the same bottle.

As a general guide, the 20 percent formula tends to suit robust, oilier or more resilient skin that tolerates active ingredients well and wants the fullest brightening and antioxidant effect. The 15 percent is a comfortable middle ground for normal to combination skin, and it's often where Dr Yasmin Tayeb starts people who are new to a higher-strength vitamin C. The 10 percent is the gentlest of the three, well suited to drier, more sensitive or reactive skin, and to anyone easing into the ingredient for the first time.

Acne-prone skin needs a little extra thought. It can be more reactive than it looks, and it's often drier than people expect, particularly if you're already using ingredients like retinoids or acids that can leave the barrier more delicate. For those patients Dr Yasmin Tayeb frequently recommends the 10 percent, at least to begin with, so the skin gets the benefits of vitamin C without being tipped into irritation.

Signs you've gone too strong

If a serum stings for more than a moment, leaves the skin looking red, or causes tightness and flaking that doesn't settle, that's usually a sign the concentration is too high for your skin right now. It doesn't mean vitamin C isn't for you. It means the strength needs adjusting. Dropping down a level, or applying every other morning until your skin adapts, often solves the problem without giving up the ingredient altogether.

Why your serum should change with the seasons

Here's the point that surprises most people. The right vitamin C for you in July may not be the right one in January. Skin behaves differently through the year, and a serum that felt perfect in summer can feel too much once the weather turns.

As autumn and winter arrive, the combination of cold air outside and dry central heating indoors pulls moisture out of the skin. The barrier becomes more fragile, and many people find their complexion turns drier, more dehydrated and a little more sensitive than usual. A higher-strength active that your skin shrugged off in warmer months can suddenly feel harsh against that backdrop.

That's why, as the colder seasons come in, Dr Yasmin Tayeb will often move a patient down a level. Someone comfortably using 20 percent through spring and summer might drop to 15 percent for the winter. An acne-prone patient whose skin tends towards dryness might come down to 10 percent, just for the coldest months. Then, as spring returns and the heating goes off, she brings them back up. It's a small adjustment that keeps the skin glowing without pushing it past its comfort zone.

Supporting your skin through the cold months

Stepping down the percentage is one part of a winter routine. Pairing your vitamin C with the right moisturiser is the other. In the colder months it helps to seal the serum in with a richer, more emollient moisturiser to lock hydration into the skin. Ingredients like hyaluronic acid and vitamin E give the complexion extra water and comfort, and they sit happily alongside vitamin C. If your skin still feels dry despite a lower-strength serum, the answer is often more moisture on top rather than abandoning the vitamin C.

How to use vitamin C well

Application is simple, but the order and timing make a difference. Vitamin C is best used in the morning on clean skin, before your moisturiser and always before your SPF. A few drops are plenty for the whole face. Let it absorb for a moment, then follow with moisturiser and sunscreen.

If you're new to the ingredient or you've had sensitivity before, start slowly. Every other morning for the first week or two lets the skin acclimatise, and you can build to daily use once you know it agrees with you. Consistency is what delivers results. Vitamin C isn't a quick fix that transforms the skin overnight. Most people notice brighter, more even skin over a matter of weeks to a few months of steady use.

Because vitamin C is fussy about light and air, treat the bottle with a little care. Close the lid properly, keep it out of direct sunlight, and don't leave it sitting on a sunny windowsill or a steamy bathroom shelf. Stored well, a good serum will stay effective for the life of the bottle.

Common questions about vitamin C serums

Is a higher percentage of vitamin C always better?

No. The benefits of vitamin C plateau at around 20 percent, and going higher tends to cause irritation without improving results. The best percentage for you is the strongest one your skin tolerates comfortably, which depends on your skin type and the season rather than on chasing the biggest number.

Can I use vitamin C if I have oily or acne-prone skin?

Yes. Vitamin C suits most skin types, and its brightening effect is welcome for the pigmentation that acne can leave behind. Acne-prone skin can be drier and more reactive than expected, though, so Dr Yasmin Tayeb often suggests a lower-strength formula such as 10 percent, particularly in winter or if you're also using retinoids or acids.

Should I use vitamin C in the morning or at night?

Morning is ideal. Vitamin C works alongside your sunscreen to defend against daytime free radical damage from UV light and pollution, so applying it before your SPF makes the most of both. It's perfectly safe at night too, but the daytime protective benefit is the reason most people use it in the morning.

How long before I see results?

Give it time and consistency. Brightening and a more even tone usually become noticeable over several weeks, with the collagen-supporting benefits building over a few months of regular use. If you stop and start, you'll wait longer to see the change.

Can I use vitamin C with other actives like retinol?

Generally yes, and many people use vitamin C in the morning and retinol at night, which keeps the two apart and reduces the chance of irritation. If your skin is sensitive or you're layering several actives, it's worth having your practitioner map out a routine so the ingredients support rather than fight each other.

Find your right vitamin C at Wellface

The difference between a serum that transforms your skin and one that irritates it often comes down to a single percentage point and the time of year you're using it. Rather than guess, it's worth having someone look at your skin and match you properly, then adjust as the seasons change.

Dr Yasmin Tayeb and the team at Wellface can assess your skin type, talk through your concerns and recommend the vitamin C strength that will actually suit you, along with the rest of a routine built around your skin. To arrange a skincare consultation at our Cobham, Esher or Wimbledon clinics, get in touch with the team and we'll help you find the right products, whether you'd like to collect them from clinic or have them sent to you.