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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.
Claritin (OTC) vs. Clarinex(prescription). Clarinex is faster acting than Claritin, both are without drowsiness. Zyrtec(OTC) ads claim it works faster than Claritin(OTC) which may be true, but Clarinex is fast acting and Zyrtec doesn't make that claim against Clarinex.