WITH REPEATED DOSING, BILE URSODEOXYCHOLIC ACID CONCENTRATIONS REACH A STEADY STATE IN ABOUT 3 WEEKS. ALTHOUGH INSOLUBLE IN AQUEOUS MEDIA, CHOLESTEROL CAN BE SOLUBILIZED IN AT LEAST TWO DIFFERENT WAYS IN THE PRESENCE OF DIHYDROXY BILE ACIDS. IN ADDITION TO SOLUBILIZING CHOLESTEROL IN MICELLES, URSODIOL ACTS BY AN APPARENTLY UNIQUE MECHANISM TO CAUSE DISPERSION OF CHOLESTEROL AS LIQUID CRYSTALS IN AQUEOUS MEDIA. THUS, EVEN THOUGH ADMINISTRATION OF HIGH DOSES (E.G., 15-18 MG/KG/DAY) DOES NOT RESULT IN A CONCENTRATION OF URSODIOL HIGHER THAN 60% OF THE TOTAL BILE ACID POOL, URSODIOL-RICH BILE EFFECTIVELY SOLUBILIZES CHOLESTEROL. THE OVERALL EFFECT OF URSODIOL IS TO INCREASE THE CONCENTRATION LEVEL AT WHICH SATURATION OF CHOLESTEROL OCCURS.
THE VARIOUS ACTIONS OF URSODIOL COMBINE TO CHANGE THE BILE OF PATIENTS WITH GALLSTONES FROM CHOLESTEROL-PRECIPITATING TO CHOLESTEROL-SOLUBILIZING, THUS RESULTING IN BILE CONDUCIVE TO CHOLESTEROL STONE DISSOLUTION.