The crystal polymorphism of Norfloxacin and its hydrates has been revisited in order to update the knowledge about the solid state behavior of this active pharmaceutical ingredient. We have re-examined the crystal structure of anhydrous Form A and clarified some ambiguities previously published, confirming that one of the two previously reported anhydrous forms A is in fact a sesquihydrate form. Moreover, the structures of two new polymorphs of the sesquihydrate have been determined by means of single crystal X-ray diffraction, and their thermodynamical relative stability has been established.
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