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The effect of humidity on the physicochemical properties of amorphous forms of cimetidine was investigated using differential scanning calorimetry, isothermal microcalorimetry, and x-ray diffraction analysis. Amorphous forms were obtained by the melting (amorphous form M [AM]) and the cotton candy (amorphous form C [AC]) methods. Thermal behaviors of AM and AC with or without seed crystals were measured using an isothermal microcalorimeter under various conditions of relative humidity (RH) and temperature, respectively. The crystallization kinetics of amorphous solids was analyzed based on 10 kinds of solid-state reaction models. AM transformed into form A at 11% RH, 50°C but transformed into a mixture of form A and monohydrate at 51% and 75% RH at 25°C. The mean crystallization times (MCTs) of the heat flow curve of AM and AC at 11% RH, 50°C were 47.82 and 32.00 hours, respectively, but at 11% RH, 25°C both were more than 4320 hours. In contrast, AC transformed into form A under all storage conditions. The MCTs of AC at 51% and 75% RH were 29.61 and 11.81 hours, respectively; whereas the MCTs of AM were 46.79 and 15.52 hours, respectively. The crystallization of amorphous solids followed the three-dimensional growth of nuclei (Avrami equation) with an induction period (IP). The IP for AM at 11% RH, 50°C was more than 2 times that for AC, but the difference in the crystal growth rate constant (CR) between AC and AM was within 10%. The IP for AM at 75% RH, 25°C was reduced to only 10% of the IP at 51% RH with increasing humidity, but the CR did not change significantly. In contrast, the IP for AC was slightly reduced at 75% RH compared with 51% RH, but the CR was about 5 times greater. At 75% RH, 25°C, the IP and CR of AM were about one-fourth the values of AC. This result suggests that the crystallization process consists of an initial stage during which the nuclei are formed and a final stage of growth.