Many active pharmaceutical ingredients (APIs) show phonon modes in the terahertz (THz) band, which are dependent on the crystallinity of the API, and are absent for an amorphous form. Thus, THz spectroscopy can be used to gain deeper understanding of crystallization processes and the stability of amorphous APIs. Here we report on the in-situ monitoring of the isothermal recrystallization of nifedipine as a model API. Starting from an amorphous form, crystallization to a metastable polymorph can be observed, followed by a transformation into the stable crystalline form.