Retinoids include vitamin A (retinol) and its metabolites, such as retinal, retinyl esters, and retinoic acid（retinol itself is the parent compound, not a metabolite）. Among these, retinoic acid is the most irritating and is typically used as a prescription drug, while the others can be used in skincare products. Regardless of the type of retinoid, they ultimately exert their effects on the skin after being metabolized into retinoic acid.