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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.
The primary difference between Claritin versus Clarinex is that one (Claritin) can be found over the counter, while the other is available by prescription only. (Click Claritin and Clarinex for more information about these two medications, including other similarities and differences.).