A stable water/liquid paraffin system water-in-oil-in-water (W/O/W) multiple emulsion was prepared by the two-step procedure of emulsification using a variety of nonionic emulsifying agents, such as Span 80 and Tween 20. After comparison of the releasing properties of such water soluble drugs as cefadroxil, cephradine, 4-aminoantipyrine and antipyrine which were entrapped separately in the inner aqueous phase of the W/O/W multiple emulsion, a large difference was observed. It was ascertained that the difference in these releasing properties was due to no physical rupture by the microscopic observation and the results of the release test of W/O/W multiple emulsion with two kinds of drugs entrapped simultaneously in an inner aqueous phase. This reason was presumed to be dependent on permeation in the oily phase of the drug itself. It was proved that the differences of releasing properties tended to depend on the molecular weight and were closely related to the drug concentration of outer aqueous phase of W/O/W multiple emulsion containing the drug in both aqueous phases prepared as an experimental model. Therefore, two possible mechanisms for the releasing of drugs in W/O/W multiple emulsion may be interpreted as follows : the first is that the mixed and inversed micelles formed by Span 80 and Tween 20 agents in the oily phase act as a carrier of drugs, and the second is that drug molecules diffuse through small pore existing in very thin lamella of the emulsifying agents partially formed in the oil layer owing to the fluctuation of the thickness.
