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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.
It should be noted that preformed vitamin A (such as retinol and other similar compounds) is much more likely to produce side effects, compared to provitamin. A sources (such as beta-carotene).