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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, the drug known generically as retinoic acid or tretinoin, is derived from vitamin A. Retinol, a less potent form of Retin-A, also has some strong science to support its effectiveness. Skin cells contain retinoid receptors that help regulate how the cell functions.