What concentration of vitamin C is most effective for the skin?

Did you know? The vitamin C has been known as ascorbic acid only since 1931. It was the scientist Szent-Györgyi who coined the name—a contraction of “anti-scorbutic” to emphasize its preventive role against scurvy. He was honored for this discovery with the Nobel Prize in Physiology or Medicine in 1937.

Since then, vitamin C has become a major active ingredient in cosmetics and dermatology, recognized for its many properties. It is a powerful antioxidant that, in addition to neutralizing the free radicals, stimulates collagen synthesis and evens out skin tone by inhibiting the melanogenesis. These various effects make vitamin C a versatile active ingredient for improving skin quality and slowing the photoaging. Today, it is found in numerous products, including face creams, serums, and targeted eye-contour treatments, where it is used at various concentrations, typically ranging from 5% to 25%.

However, despite the wide range of formulations available on the market, it remains difficult to assert that one concentration is objectively "better" than another.

The currently available studies do not allow us to reliably compare the different concentrations of vitamin C, because no clinical trial has directly tested these percentages against each other. In other words, while we know that vitamin C is effective, we cannot say that any specific concentration works better than others for skin care.

In contrast, data in vitro provide some insights and suggest that a 20% concentration might be optimal to promote cutaneous penetration of ascorbic acid. This was specifically explored in a study using Franz diffusion cells and piglet skin, a classic model for evaluating cutaneous penetration. Researchers applied different concentrations of vitamin C (5%, 10%, 20%, and 25%) in a lotion and, after 24 hours, measured the actual amount absorbed into the epidermis, dermis, and receptor compartment via HPLC analysis.

The results demonstrate a clear concentration dependence: the 20% and 25% formulations penetrate most effectively, with 62.3 mg and 64.6 mg of vitamin C absorbed, respectively, and the 20% concentration even reaches a diffusion rate of about 84.7%, or 1.5 times that of the 10% formula. Interestingly, increasing the concentration beyond 20% does not proportionally improve penetration: although the 25% formulation yields a higher absolute uptake, its diffusion percentage declines, suggesting that an optimal absorption threshold appears to lie around 20%. This behavior is likely linked to the skin transport mechanisms of ascorbic acid, whose diffusion capacity no longer increases beyond a certain threshold.

This does not mean that a 20% concentration of vitamin C is inherently necessary: the formulation, individual tolerance, and skin sensitivity also play important roles.

We also observe that 10% and 15% formulations penetrate the skin as well, even if their overall diffusion is lower. At the same time, it’s important to bear in mind that high concentrations of ascorbic acid increase the risks of irritation, redness, and sensitivity. The vitamin C pure form is very unstable and oxidizes rapidly upon exposure to air, light, or heat. This explains why some serums turn brown over time. To limit this oxidation, ascorbic acid must be formulated at a pH below 3.5—a more acidic environment than that of the skin—which can itself heighten the risk of irritation. This is why derivatives of the vitamin C are frequently used in cosmetics.

Moreover, some data indicate that a 10% concentration of vitamin C can already yield significant results in photoaging. A clinical study involving ten patients assessed, in a double‐blind design, the application of a complex combining 10% ascorbic acid, which is water soluble, and 7% tetrahexyldecyl ascorbate, a derivative of vitamin C that is lipid soluble, applied to one half of the face, while the other half received only the vehicle. After 12 weeks, photoaging scores decreased significantly on the treated side. Skin biopsies showed a increase in collagen and an elevation in type I collagen mRNA expression, confirming the stimulation of fibroblast activity.

This study demonstrates that a well-designed formulation, even at 10%, can induce measurable structural changes and a clinical improvement in wrinkles after 12 weeks of application.

Note : If this is your first time using a pure vitamin C treatment, we recommend starting with a low concentration (≈ 5%) and applying it every other day. If your skin tolerates the active ingredient well, you can switch to daily application and then increase the concentration.

• FITZPATRICK R. & al. Double-blind, half-face study comparing topical vitamin C and vehicle for rejuvenation of photodamage. Dermatologic Surgery (2002).

• CHIANG N. & al. Topical vitamin C and the skin: Mechanisms of action and clinical applications. Journal of Clinical and Aesthetic Dermatology (2017).

• LIU Q. & al. Study on the efficacy of vitamin C lotion on skin: Permeable and anti-aging. Journal of Cosmetics, Dermatological Sciences and Applications (2022).

• MAGINA S. & al. Efficacy of topical vitamin C in melasma and photoaging: A systematic review. Journal of Cosmetic Dermatology (2023).