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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.
Retinol is an active form of vitamin A, a fat-soluble vitamin that is essential to vision and bone development. Sources include liver, whole milk, and fortified foods. The body can also convert it to retinal and retinoic acids as found in carotene compounds.