What are the side effects of vitamin C?

Ascorbic acid, a vitamin with potential risks?

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

Let us clarify from the outset that vitamin C is not a hazardous active ingredient.

However, when used at high concentrations, particularly in its pure form (L-ascorbic acid), tingling sensations, redness, or mild irritation are relatively common, especially in sensitive skin or skin with a skin barrier weakened. Interestingly, there is an intriguing paradox: users regularly report discomfort, yet the scientific literature does not document as many cases of irritation as one might expect.

A clinical study illustrates this point particularly well. Thirty-four women participated in an occlusive patch test 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 kept under the patch for 30 minutes, 24 hours, and 48 hours. After removing the patch, the skin was examined. Result: no erythema, no dryness, and no edema were observed throughout the evaluation period. The results were thus negative, demonstrating that the lotion containing 20% vitamin C did not induce irritation or allergic reaction under these experimental conditions. This type of test suggests that ascorbic acid is not necessarily irritating by nature, but that reactions observed in everyday use likely stem from other factors: a fragile skin barrier, an accumulation of exfoliants, a very acidic 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 oxidizes rapidly upon contact with 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 color change indicates molecular degradation accompanied by a significant loss of antioxidant activity. To slow this oxidation, ascorbic acid must be formulated at a pH below 3.5. However, this pH is more acidic than that of the skin (around 5.5–6) and can promote irritation.

To overcome this issue, the cosmetics industry often uses stabilized derivatives of vitamin C (sodium ascorbyl phosphate, ascorbyl tetraisopalmitate, ascorbyl palmitate), which reduces the risk of irritation.

4 minutes to understand your skin. Our dermatological diagnostic guides you toward the ideal skincare for your specific needs. Simple, quick, personalized.

Precautions to consider when using vitamin C?

Vitamin C has no specific contraindications for topical application, nor are there strict usage precautions. It can be used on all skin types, including sensitive skin. However, when the formulation contains pure ascorbic acid at high concentrations (15–20%), such skin types may experience tingling or warming sensations. In this case, it is often preferable to begin with concentrations of vitamin C that are more moderate or to switch to stabilized derivatives, better tolerated and formulated at pH levels closer to that of the skin.

There is also no contraindication to the use of vitamin C in pregnant or breastfeeding women when applied to the skin. This distinction is important, as guidance differs when it comes to oral administration. Indeed, the use of vitamin C as supplements must comply with the recommendations of the physician or healthcare professional overseeing the pregnancy. When applied topically, its use is considered safe.

Moreover, one often attributes vitamin C photosensitizing 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 has a photoprotective action thanks to its potent antioxidant power. It does not filter UV like a sunscreen, but neutralizes free radicals notably generated by UV rays, thus helping to limit photodamage.

Vitamin C does not replace sunscreen, but acts as a supplement.

Several scientific studies have investigated the ability of the vitamin C to boost the skin’s defense against UV radiation, particularly when combined with other antioxidants such as vitamin E. One particularly noteworthy study assessed the photoprotective efficacy of a formulation containing 15% L-ascorbic acid and 1% α-tocopherol, compared to the same concentrations used separately. For this purpose, researchers applied the antioxidant solutions to pig skin for four days, then exposed the treated areas to a calibrated solar simulator (1 to 5 minimal erythema doses, corresponding to the minimum dose needed to induce sunburn). The results showed that vitamin C alone, as well as vitamin E alone, offered some protection against erythema and the formation of 'sunburn cells', while the combination of the two vitamins was markedly superior.

Irradiation de la peau avec vitamines C et E (C&E) ou le véhicule (VEH) et mesure de la capacité antioxydante. — Skin irradiation with vitamins C and E (C&E) or the vehicle (VEH) and measurement of antioxidant capacity.
Source: PINNELL S. R. & al. UV photoprotection by combination topical antioxidants vitamin C and vitamin E. Journal of the American Academy of Dermatology (2003).

Note : It is not recommended to use products containing pure vitamin C at the same time as others containing potent actives such as alpha-hydroxy acids (lactic acid, glycolic acid), azelaic acid, salicylic acid and retinol.