Why does the skin become darker when using retinol?

After using retinol, the skin may darken primarily because retinol (i.e., vitamin A alcohol) is photosensitive and easily oxidized by ultraviolet (UV) light. When skincare products containing retinol are applied and the skin is subsequently exposed to sunlight immediately, the retinol can be oxidized and decomposed by UV radiation. The decomposition products, together with UV light, can irritate the skin, increase skin sensitivity, and promote melanin deposition, resulting in skin darkening. Therefore, after applying skincare products containing retinol, one should avoid direct exposure to sunlight and take sun protection measures, such as applying sunscreen and using an umbrella, to reduce UV damage to the skin and prevent skin darkening. Additionally, it is important to choose qualified and legitimate retinol skincare products and avoid excessive use to minimize potential harm to the skin.