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When a large quantity of insoluble drugs was added into a solution containing a small amount of soluble drugs, the latter was adsorbed to the former. Their adsorption was expected to alter the drug activity. Berberine hydrochloride (BH) was selected as a soluble drug and tested with frequently combined insoluble drugs and antacids for their adsorption phenomena in purified water, gastric and intestinal fluid test solutions, respectively. The. adsorption isotherms of kaolin and -natural aluminum silicate with BH in the three media fitted the Langmuir (LM) equation, and that of talc in purified water and gastric fluid fitted it, but in intestinal fluid it fitted the Freudlich equation. The adsorption isotherm of aluminum hydroxide fitted the LM equation only in intestinal fluid. The degree of adsorption of BH in purified water and gastric fluid is in the following order: magnesium trisilicate, kaolin, natural aluminum silicate and talc; in intestinal fluid: magnesium trisilicate, kaolin, talc, natural aluminum silicate and aluminum hydroxide. Magnesium stearate did not adsorb BH.
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