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The two are often believed to be interchangeable, but the main difference is that retinol is eventually converted into retinoic acid. Retinol is a natural form of vitamin A, while Retin-A is derived from retinoic acid, a synthetic derivative of vitamin A, and considered pharmaceutical.
Retinoids come in prescription form and in a range of over-the-counter products. Prescription-level retinoids fall into these groups: Tretinoin, including the brands Atralin, Retin-A, Retin-A Micro, and Renova; Tazarotene, such as the brand Tazorac Adapalene, such as the brand Differin