The hazards of retinol.

The retinol is a potent active ingredient capable of acting on wrinkles, brown spots, and blemishes. These various actions come notably from its ability to stimulate cellular renewal. However, this effectiveness of the retinol comes with a significant risk of skin irritation, particularly during the first weeks of use and in sensitive skin. Indeed, it is not uncommon to notice redness and slight skin peeling when starting to use a product containing retinoids.

This irritation process is linked to several biological mechanisms. On one hand, retinol accelerates the renewal of keratinocytes, the cells of the stratum corneum, which temporarily disrupts the skin barrier and reduces the amount of certain protective intracellular lipids, such as ceramides or cholesterol. This alteration of the barrier function also increases transepidermal water loss, which heightens the skin's vulnerability to external irritants and environmental allergens and can make it dehydrated. Retinol can also trigger a mild inflammatory reaction, characterized by the release of mediators, such as interleukin-8 (IL-8) and tumor necrosis factor alpha (TNF-α).

How to counteract the irritation associated with retinol?

Retinol-induced irritation is often temporary and does not necessarily require discontinuation of its use. It is often sufficient tospace out the applications, using it every other night, or even every third night, depending on skin tolerance. Additionally, it is also possible to start at a low dose, such as 0.01% or 0.05%, and gradually increase the concentration of retinol. Lastly, the method of the "retinol sandwich", which involves applying the retinol treatment between two layers of moisturizer, can help reduce the risk of irritation while gradually acclimating the skin to this active ingredient.

While the photosensitizing nature of retinol has not been clearly established, this molecule nonetheless remains unstable in light. Indeed, due to the numerous double bonds in its chemical structure, when retinol is exposed to UV rays, it is likely to undergo chemical degradation and transform into reactive intermediates. By oxidizing, retinol is responsible for the release of free radicals on the skin's surface. These reactive species can damage cell membranes, proteins, and DNA, promoting redness and burning sensations, symptoms similar to those of a sunburn. That's why it's recommended to use retinol treatments in the evening and apply sun protection the following morning.

This precaution applies to all retinoids (retinal, tretinoin, adapalene, etc.).

One of the most frequently reported transient side effects with the use of retinol is the skin purge, also known as the rebound effect. This phenomenon manifests as a pimple outbreak, similar to an acne flare-up, which occurs in the first few weeks of application. Although frustrating, the purge is actually a reflection of retinol's effectiveness. It follows a stimulation of cell renewal, which explains why active ingredients such as retinoids, AHAs, BHAs, or PHAs can trigger it. Indeed, by accelerating the skin regeneration process, these molecules allow the deep cells of the epidermis to rise to the surface more quickly, bringing with them imperfections that are still invisible. Without the purge, the skin would have expelled impurities more slowly, one by one over time.

The duration of the purge varies among individuals, but it generally resolves within 4 to 8 weeks. To limit the intensity of this reaction, we advise you to gradually introduce the retinol into your routine, starting with a low concentration and spacing out the applications.

Among the most serious dangers associated with the use of retinol is its teratogenic potential, that is, its ability to cause congenital malformations in the fetus. This risk is well-documented for oral retinoids, such as isotretinoin, but also applies, as a precautionary principle, to topical retinoids, like retinol or tretinoin.

Even though the skin's absorption of retinol is very low, this molecule can potentially enter the bloodstream. Retinoids are known to interfere with the signaling pathways of Vitamin A, which plays a crucial role in embryonic development, particularly in the central nervous system. Even minimal exposure, especially during the first trimester, poses a theoretical risk of causing severe abnormalities.

All retinoids, including those used in skincare, are strictly contraindicated for pregnant women, nursing mothers, or those planning to become pregnant.

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• MAIBACH H. I. & others The distinctiveness of irritation caused by retinoids and its significance in clinical effectiveness. External Dermatology (2002).

• KANG K.-S. et al. The process of retinol-induced irritation and its use in the development of anti-irritants. Toxicology Letters (2003).

• STANFIELD J. & others Reevaluating the phototoxicity of tretinoin: a report of four controlled clinical trials. Photodermatology, Photoimmunology & Photomedicine (2009).

• ZASADA M. & al. Retinoids: Active Molecules Influencing Skin Structure Formation in Cosmetic and Dermatological Treatments. Advances in Dermatology and Allergology (2019).

• KORE S. & others. Retinoids — A Singular Component for Skin Revitalization Utilizing Innovative Drug Delivery Systems. Journal of Applied Pharmaceutical Science (2024).