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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 is available in prescription-strength products, including generic formulations, and under brand names like Retin-A Micro, Renova, Avage and Tazorac. Its less potent over-the-counter cousin, retinol, is found in products on department and drugstore shelves.