Water activities for the binary aqueous solution of ionic liquid of 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate ([BMP][TfO]), and also three ternary aqueous solutions containing ionic liquid of [BMP][TfO] and drugs of diphenhydramine-hydrochloride, guaifenesin and naproxen sodium were measured using an improved isopiestic method to study the medicines effect on water activity of ionic liquid aqueous solution at T = 298.15 K. Vapor pressure values were calculated from the water activity data measured in this work. NRTL, NRF-NRTL, mNRTL, Wilson and TNRF-mNRTL local composition based models were applied for correlating the water activity data for binary and ternary aqueous solutions containing ionic liquid and drugs considered in this work. All of these models satisfactorily predicted the water activity data for the systems investigated. Water activity data and vapor pressure decreasing were utilized to highlight the solute-solvent interactions.