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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.
Tretinoin (brand names Retin-A, Avita, Renova) is a derivative of vitamin A and is the treatment of choice for comedonal acne as well as whiteheads and blackheads. It works by increasing skin cell turnover which promotes the extrusion of the plugged material in the follicle.