The dark side of vitamin C

​Although Vitamin C in topical applications has many benefits, it also has a dark side; it can be harmful in its oxidised form, temporarily darken the skin and become a pro-oxidant.

When vitamin C (ascorbic acid) is exposed to air, light, or heat, it undergoes chemical changes similar to how sugar turns brown when heated. This process doesn't need any special helpers (like enzymes); it just happens because of the conditions around it. Over time, vitamin C breaks down and forms new compounds that have a brown color, much like how sugar becomes caramel. This process is called non-enzymatic oxidation.
 
Oxidized vitamin C can have both beneficial and potentially harmful effects on the skin.

1. ANTIOXIDANT
Vitamin C is primarily known for its antioxidant properties, effectively neutralizing reactive oxygen species (ROS) and reducing oxidative stress in the skin. This helps prevent DNA damage and collagen degradation, contributing to anti-aging benefits and improved skin health and beauty [1][2][3].
 
How vitamin C acts as an antioxidant and undergoes oxidation in your skin
Imagine vitamin C as a brave knight patrolling your skin, constantly on guard against harmful invaders called free radicals. These free radicals can damage skin cells, much like how rust can damage metal. Vitamin C, in its role as an antioxidant, sacrifices part of itself (donating an electron) to neutralize these free radicals, preventing them from causing harm.

▌ InInitial defense: When vitamin C donates an electron, it transforms into a less powerful form called the ascorbate radical, similar to a knight losing a piece of armor but still able to fight.
▌ Continued protection: If more free radicals attack, vitamin C can further degrade into dehydroascorbic acid. This form can be regenerated with the help of other antioxidants like glutathione, similar to allies helping the knight repair its armor.
▌ Synergistic effects: Using vitamin C with other antioxidants in skincare products enhances its protective abilities, much like having a team of knights working together for stronger defense. I prefer combining Vitamin C with Licochalcone A for comprehensive skin protection. Vitamin C acts quickly in the skin's outer layer, providing immediate extracellular defense. Meanwhile, Licochalcone A offers long-lasting, intracellular protection against free radicals induced by both UV and High Energy Visible Light, which penetrate deeper into the skin. This synergistic approach ensures a more complete and sustained antioxidant effect.
▌ Final sacrifice: Without support, vitamin C eventually breaks down into other compounds and loses its protective power completely.

2. PRO-OXIDANT At high concentrations, vitamin C can exhibit pro-oxidative properties, generating hydrogen peroxide (H2O2) and leading to increased oxidative stress, particularly when vitamin C interacts with transition metals (Cu and Fe), which can catalyze the formation of harmful radicals [4][5]. This increases the risk of irritation or damage to skin cells.
 
Copper (Cu): Copper compounds can penetrate the skin and participate in redox reactions [6]. Copper can catalyze the oxidation of ascorbate and participate in the Haber-Weiss reaction, generating free radicals [7].
Iron (Fe): Iron can participate in the metal-catalyzed Haber-Weiss reaction, also known as the superoxide-driven Fenton reaction, which produces harmful free radicals [7].
 
These transition metals can contribute to oxidative stress in the skin through the following mechanisms:
 
▌ Catalyzing the production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) [8].
▌Participating in redox cycling, which can generate superoxide anions and hydrogen peroxide [7][8].
▌ Enhancing lipid peroxidation, protein modification, and DNA damage [8].
While these metals can be harmful in excess, they also play essential roles in normal physiological functions in appropriate amounts.

3. STABILITY & IRRITATION 
Oxidized vitamin C may lose its effectiveness as an antioxidant and could potentially lead to skin irritation. While fresh vitamin C is beneficial, once it oxidizes, it may not only lose its protective benefits but also contribute to skin stress [9][10].

4. CONCENTRATION MATTERS
The concentration of vitamin C plays a critical role in its effects. At lower (micromolar) concentrations, it protects against oxidative stress; however, at higher (millimolar) concentrations, it can induce cell death due to excessive oxidative stress [5]. 

Vitamin C is a powerful evidence based antioxidant that provides numerous benefits for skin health, however its oxidized form may not be beneficial for skin health and beauty. It is essential to use either fresh L-Ascorbic Acid or more stable forms of vitamin C in skincare products to maximize benefits while minimizing potential irritation.
 
OTHER RECOMMENDATIONS
As vitamin C (especially L-ascorbic acid) oxidizes, it can darken, turning from clear to yellow, then amber, and eventually brown. 
 
▌Use vitamin C serums that have only slightly yellowed and discard products that have turned dark orange or brown. Be aware of signs of oxidation, such as changes in color or smell. 
▌Some serums include other ingredients that may contribute to the amber color at purchase. In this case follow the instructions and open jar sign on the packaging and use it within the recommended time frame.
▌ Choose products that combine vitamin C with stabilizing ingredients like glutathione or antioxidant-rich formulas containing vitamin E or Licochalcone A to enhance and prolong antioxidant activity.
▌Store your vitamin C serum properly (cool, dark place. Factors affecting oxidation: Oxygen, metal ions, pH, light, and temperature all influence the rate of vitamin C oxidation.
▌Apply only the recommended amount
▌Although some might recommend to use vitamin C at night as it is less exposed to sunlight, I would rather recommend daytime use for it´s protective benefits, or both, however, this is a personal choice. Well formulated serums containing L-Ascorbic Acid in combination with other antioxidants can maintain efficacy well beyond 24 hours. Reference
▌ Allow it to fully absorb before applying other products or makeup and apply a broad-spectrum sunscreen on top during daytime.
 
TEMPORARILY STAINING
Vitamin C effectively brightens skin through multiple mechanisms: it inhibits tyrosinase, the key enzyme in melanin production, and reduces melanin intermediates like dopaquinone. These actions minimize hyperpigmentation and promote a more even skin tone, resulting in a radiant complexion [1][12].
 
However, vitamin C can also darken the skin temporarily. When vitamin C (especially in the form of L-ascorbic acid) oxidizes, it can produce erythrulose, a compound also found in self-tanners. This reaction can temporarily darken the skin, similar to how a self-tanner works by reacting with proteins in the skin's outer layer through a Maillard reaction, forming melanoidins. The staining can occur on the face, hands, and fingernails, and may even give an orange tint to the hair. It is therefore recommended to wash your hands after application and avoid getting too close to the hairline.
 
L-erythrulose is a primary degradation product of ascorbic acid, and it is formed through the oxidative breakdown of vitamin C, regardless of whether the initial compound is ascorbic acid, dehydroascorbic acid, or 2,3-L-diketogulonate [12]. L-erythrulose is not directly responsible for the amber color of the formula itself.
 
Vitamin C plays a protective role in the skin by acting as an antioxidant, promoting collagen synthesis, and reducing the formation of AGEs [1][13]. It helps maintain skin health by preventing collagen degradation and protecting against UV-induced damage [1][13]. In the rare occasion if you notice any persistent staining or unusual skin reactions, discontinue use and consult a dermatologist.
 
Take care
Anne-Marie
 
References
[1] Al-Niaimi F, Chiang NYZ. J Clin Aesthet Dermatol. 2017 Jul;10(7):14-17.
[2] Khalid A, et al. J Health Rehabil Res. 2024;4(2):1489-1494.
[3] Pullar JM, et al. Nutrients. 2017 Aug 12;9(8):866.
[4] Kaźmierczak-Barańska J, et al. Nutrients. 2020 May 21;12(5):1501.
[5] Chakraborty A, Jana NR. ACS Appl Mater Interfaces. 2017 Dec 
[6] Hostynek JJ, Maibach HI. Toxicol Mech Methods. 2006;16(5):245-65.
[7] Buettner GR, Jurkiewicz BA. Radiat Res. 1996 May;145(5):532-41.
[8] Chaudhary P, et al. Front Chem. 2023 May 10;11:1158198.
6;9(48):41807-41817.
[9] Jelodar G, et al. Zahedan J Res Med Sci. 2023;25(4):e4037.
[10] Podmore ID, et al. Nature. 1998 Apr 9;392(6676):559.
[11] De Dormael R, et al. Vitamin C Prevents UV Pigmentation: Meta-analysis. J Clin Aesthet Dermatol. 2019;12(2):E53-E59. 
[12] Simpson GL, Ortwerth BJ. Biochim Biophys Acta. 2000;1501(1):12-24.
[13] Wang K, et al. Role of Vitamin C in Skin Diseases. Front Physiol. 2018;9:819.

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