Potential risks of vitamin C?

The vitamin C is one of the most popular active ingredients in cosmetics, frequently used to even out skin tone, support collagen synthesis and protect the skin from oxidative stress. It can be found at various concentrations, generally between 5 and 20%, although its maximum concentration is not regulated.

It should be noted from the outset that vitamin C is not a hazardous active ingredient.

In contrast, when it is applied at high concentrations, notably in its pure form (L-ascorbic acid), sensations of tingling, redness or mild irritation are relatively common, particularly in sensitive skin or where there is a skin barrier that is compromised. There is, indeed, an interesting paradox: user experiences regularly report discomfort, yet the scientific literature does not document as many cases of irritation as one might anticipate.

A clinical study illustrates this point particularly well. 34 women participated in an occlusive patch test designed to assess the irritant potential of a lotion containing 20% of vitamin C. The forearm area was shaved four hours before application, and the product was left under the patch for 30 minutes, 24 hours and 48 hours. After removal of the patch, the skin was examined. Result: no erythema, no dryness, nor any oedema were observed throughout the evaluation period. The results were therefore negative, showing that the lotion containing 20% vitamin C did not elicit irritation or an allergic reaction under these experimental conditions. This type of test suggests that ascorbic acid is not necessarily irritating by nature, but that the reactions observed in everyday life likely stem from other factors: a compromised skin barrier, accumulation of exfoliants, very low pH or overuse.

However, there are some cases of skin sensitization by vitamin C. One example is the case of a 47-year-old woman who developed facial eczema for three months, initially located in the eyelids, then extended to the entire face and neck. Patch tests revealed a positive reaction to a cream used before the appearance of lesions. Tests were then conducted with each ingredient and researchers identified vitamin C as responsible. The cessation of cream use resulted in a complete cure without relapse. This is therefore a case of contact dermatitis due to vitamin C. Nevertheless, the study does not specify at any time the concentration of vitamin C used, which limits the scope of the results.

One essential point remains: the vitamin C in its pure form is intrinsically unstable, as it is highly sensitive to oxidation. In aqueous solution, ascorbic acid rapidly oxidises on exposure to light, oxygen, light or heat. This is why pure vitamin C serums can darken, shifting from a pale yellow to a deep orange or even brown. This colour change indicates molecular degradation, which is accompanied by a marked reduction in its antioxidant activity. To slow this oxidation, ascorbic acid must be formulated at a pH below 3.5. However, this pH, being more acidic than that of the skin (around 5.5–6), can promote irritation.

To overcome this issue, the cosmetics industry frequently employs stabilized derivatives of vitamin C (sodium ascorbyl phosphate, ascorbyl tetraisopalmitate, ascorbyl palmitate), thereby minimising the risk of irritation.

Vitamin C has no specific contraindications when applied topically, nor are there strict usage precautions. It can be used by all skin types, including sensitive skin. However, when the formulation contains pure ascorbic acid at a high concentration (15 to 20%), they may experience tingling or warming sensations. In this case, it is often preferable to start with concentrations of vitamin C more moderate ones or to turn to stabilised derivatives, better tolerated and formulated at pH values closer to that of the skin.

There is likewise no contraindication to the use of vitamin C for pregnant or breastfeeding women when applied to the skin. This detail is important, as guidance differs for oral intake. Indeed, the use of vitamin C in supplement form should be carried out in accordance with the recommendations of the doctor or healthcare professional overseeing the pregnancy. For topical application, its use is considered safe.

Furthermore, people often attribute to vitamin C photosensitising properties, but this is a misconception arising from its instability under UV light, which leads to its oxidation into dehydroascorbic acid and 2,3-diketogulonic acid, responsible for the yellowing of formulations. In reality, vitamin C possesses a photoprotective action thanks to its powerful antioxidant capacity. It does not filter UV like a sunscreen but neutralises free radicals particularly those generated by UV rays, thereby helping to limit photodamage.

Vitamin C does not replace sunscreen, but complements it.

Several scientific studies have examined the ability of vitamin C to strengthen the skin’s defence against UV radiation, particularly when combined with other antioxidants such as vitamin E. A notably interesting study assessed the photoprotective efficacy of a formulation containing 15% L-ascorbic acid and 1% α-tocopherol, compared to the same concentrations used individually. To this end, the researchers applied the antioxidant solutions to porcine skin for four days, then exposed the treated areas to a calibrated solar simulator (1 to 5 minimal erythemal doses, corresponding to the minimum dose required to induce sunburn). The results demonstrated that vitamin C alone, as well as vitamin E alone, offered a certain protection against erythema and the formation of "sunburn" cells, whereas the combination of the two vitamins was markedly superior.

Note : It is not advisable to use products containing pure vitamin C simultaneously with others that contain potent actives such as alpha-hydroxy acids (lactic acid, glycolic acid), the azelaic acid, the salicylic acid and the retinol.

• BAZEX J. & al. Contact dermatitis from vitamin C in a cosmetic anti-aging cream. Contact Dermatitis (2002).

• PINNELL S. R. & al. UV photoprotection by combination topical antioxidants vitamin C and vitamin E. Journal of the American Academy of Dermatology (2003).

• ANSARI S. A. & al. Photostability and interaction of ascorbic acid in cream formulations. AAPS PharmSciTech (2011).

• STAMFORD N. P. J. Stability, transdermal penetration, and cutaneous effects of ascorbic acid and its derivatives. Journal of Cosmetic Dermatology (2012).

• TELANG P. S. Vitamin C in dermatology. Indian Dermatology Online Journal (2013).

• CROWTHER C. A. & al. Vitamin C supplementation in pregnancy. Cochrane Database of Systematic reviews (2015).

• RICCI LEONARDI G. & al. Vitamin C: One compound, several uses. Advances for delivery, efficiency and stability. Nanomedicine: Nanotechnology, Biology, and Medicine (2020).

• 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).