Cosmetics are not miracle products!

Here is the definition of a cosmetic as per the ANSM (National Agency for the Safety of Medicines and Health Products).

"A cosmetic product is defined as any substance or mixture intended to come into contact with the superficial parts of the human body (epidermis, hair and capillary systems, nails, lips and external genital organs) or with the teeth and oral mucous membranes with the aim, exclusively or mainly, to clean them, to perfume them, to alter their appearance, to protect them, to keep them in good condition or to correct body odours.

Cosmetic products placed on the market must be safe for human health. They are not subject to prior authorisation before their market release. Therefore, it is the responsibility of the Person Responsible associated with each cosmetic product to ensure that their products meet the requirements of the cosmetic regulation as well as legislative and regulatory requirements, and to guarantee that they pose no risk to health."

Thus, cosmetics are by no means medicines! They do not have a therapeutic action and do not treat skin conditions. They do not require a marketing authorisation (MA) before they are sold, unlike medicines. The assessment of their effectiveness and tolerance is also different from that of medicines. However, they can still have a visible effect on the skin, provided that their use is accompanied by a good lifestyle and a beauty routine that is consistent with one's skin needs.

The ingredients found in a cosmetic product typically penetrate the stratum corneum, the most superficial layer of the skin, to disperse into the deeper layers. However, there must be boundaries: the active ingredients in cosmetic products should never enter the bloodstream. Their action must remain superficial and localised.

When a cosmetic product is applied to the skin, it must first penetrate the stratum corneum, which is composed solely of dead cells known as corneocytes. If it contains small-sized actives such as caffeine (molar mass = 194.2 g/mol) and/or hydrophilic compounds, it takes the so-called trans-cellular route (passage of actives from cell to cell). If it is a lipophilic care product (composed of fatty substances), the inter-cellular pathway is favoured: the actives then circulate in the inter-lipid cement that ensures cellular cohesion of the stratum corneum, and then in the interstitial fluid that fills the spaces between the cells of the deeper layers of the skin.

Generally, in cosmetics, molecules act at the level of the epidermis but do not reach, or reach in very small quantities, the dermis. For an active ingredient to diffuse to the dermis, several intrinsic and extrinsic conditions must be met:

• The molecule must be of small size, akin to caffeine or low molecular weight hyaluronic acid;

• Skin hydration should be optimal, and the skin should be neither too oily nor too dry;

• Small amphiphilic molecules, which have an affinity for both lipids and water, are the ones that penetrate the deepest;

• The thickness of the product layer applied to the skin and the frequency of application have an influence on the penetration of active ingredients.

Generally, one must wait at most four weeks before seeing the effects on the skin of a cosmetic treatment. This duration corresponds approximately to the time it takes for cells to renew themselves (we talk about the cell cycle). However, some active ingredients can act in a few days on the skin's appearance and its imbalances. This is the case, for example, with salicylic acid , which acts in three days on imperfections when it is concentrated at least at 2%. As for brightening/anti-spot treatments, it is generally advisable to wait about six weeks before noticing the effects (or not) on one's skin. Indeed, hyperpigmentation is a complex phenomenon that involves a large number of cells and biological mechanisms.