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Precipitation on the crystal surface of PIO HCl salt immediately after contact with blank FaSSIF.Precipitation of a drug on the particle surface of its salt was observed under the polarized light microscope using pioglitazone (PIO).A simple method to immobilize particles between the slide glasses was developed to observe the rapid precipitation process.Crystalline precipitates appeared on the surface of PIO HCl within 15 s after contact with biorelevant media.The crystalline precipitates were suggested to be the free base of PIO by microscopic Raman spectroscopy and powder X-ray diffraction.HPMC and HPMC-AS inhibited the precipitation on the surface of PIO HCl.The purpose of the present study was to investigate the precipitation of a drug on the particle surface of its salt in biorelevant media. Pioglitazone (PIO, weak base, pKa = 5.8) was used as a model drug. The crystal particles of PIO hydrochloride (PIO HCl) were immobilized between two slide glasses. A biorelevant medium was penetrated between the slide glasses under a polarized light microscope. Crystalline precipitates appeared on the surface of PIO HCl within 15 s after contact with the biorelevant media. The crystalline precipitates were suggested to be a free base of PIO by microscopic Raman spectroscopy and powder X-ray diffraction. Bile micelles affected the precipitation patterns on the surface. Hydroxypropylmethylcellulose and hydroxypropylmethylcellulose acetate succinate inhibited the precipitation. The precipitation on the surface of its salt could be an important factor that could affect the dissolution profiles of a drug.