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

Did you know? The vitamin C has only been known as ascorbic acid since 1931. It was named by the scientist SZENT-GYORGI – a contraction of “anti-scorbutic” to highlight its preventive role against scurvy. He was awarded the Nobel Prize in Physiology or Medicine in 1937 for this discovery.

Since then, vitamin C has become a major active ingredient in cosmetics and dermatology, renowned for its numerous properties. It is a potent antioxidant which, in addition to neutralising the free radicals, stimulates collagen synthesis and evens out skin tone by inhibiting melanogenesis. These various effects make vitamin C a versatile ingredient for improving skin quality and slowing down the photoageing. It is now found in many products, notably facial creams, serums or eye contour treatments, where it is used at different concentrations, generally ranging from 5% to 25%.

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

The studies currently available do not enable a reliable comparison of the different concentrations at which vitamin C is found, as no clinical trial has tested these percentages directly against each other. In other words, it is known that vitamin C is effective, but one cannot say that any specific concentration works better than others for skin care.

On the other hand, data in vitro provide some insights and suggest that a concentration of 20% could be optimal to favour skin penetration of ascorbic acid. This was explored in a study using Franz diffusion cells and pig skin, a classical model for assessing skin penetration. Researchers applied various concentrations of vitamin C (5%, 10%, 20% and 25%) in a lotion and, after 24 hours, measured by HPLC the amounts actually absorbed into the epidermis, dermis and receptor medium.

The results show 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 achieves a diffusion rate of approximately 84.7%, which is 1.5 times that of the 10% formulation. Interestingly, increasing the concentration beyond 20% does not proportionally enhance penetration: the 25% formulation delivers a higher absolute amount, but its diffusion percentage decreases, suggesting a threshold for optimal absorption appears to lie at around 20%. This behaviour is likely related to the cutaneous transport mechanisms of ascorbic acid, whose diffusion capacity does not increase beyond a certain threshold.

This does not mean that a 20% concentration of vitamin C is necessarily required: the formulation, individual tolerance and skin sensitivity also play a part.

It is also observed that 10% and 15% formulations penetrate the skin equally well, even if their dispersion is lower. At the same time, one should bear in mind that high concentrations of ascorbic acid increase the risks of irritation, redness and sensitisation. The pure vitamin C is extremely unstable and oxidises rapidly when exposed 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, an environment more acidic than that of the skin, which can, in itself, heighten the risk of irritation. This is why derivatives of vitamin C are frequently used in cosmetics.

Moreover, some data indicate that a 10% concentration of vitamin C can already yield significant results in photo-ageing. A clinical trial conducted on ten patients evaluated the double-blind 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 side of the face, while the other side received only the vehicle. After 12 weeks, photo-ageing scores decreased significantly on the treated side. Skin biopsies showed a increase in collagen and an increase in type I collagen mRNA expression, confirming the stimulation of fibroblast activity.

This study shows 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).