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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 in the vitamin A family. It or other forms of vitamin A are needed for eye sight, maintenance of the skin, and human development. It is converted in the body to retinal and retinoic acid through which it acts. Dietary sources include fish, dairy products, and meat.