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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.
Retin-A is referenced as being 100 times stronger than retinol. It also has a more immediate effect because it is formulated as retinoic acid; unlike retinols, no conversion by the body is required. As such, many users of Retin-A experience notable improvements in as little as 4-8 weeks.