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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.
In the most basic terms, retinol is the whole vitamin A molecule, which is one of the best vitamins for your skin. Vitamin A is a potent antioxidant, and works to protect your skin from free radicals, generates cell growth, and repairs damaged cells.