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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.
These compounds result in proliferation and reduced keratinisation of skin cells independent of their functions as a vitamin. Many brand-name creams containing the retinoids retinol and retinaldehyde can be obtained over the counter at pharmacies and supermarkets. The more potent topical retinoids available on prescription are: Tretinoin; Isotretinoin; Adapalene; Alitretinoin.