Nachajski et al.: Role of API in the Process of Direct Compression The aim of the study was to analyse the effect of the active pharmaceutical ingredient on the fl owability of the powder and properties of tablets prepared using direct compression method. Each analysed formulation contained one model active pharmaceutical ingredient, diphenhydramine, ketoprofen or loratadine. Granulometric properties of the powder formulations were determined based on bulk density prior and after compression, Hausner ratio, Carr index and angle of repose values. The analysis of manufactured tablets included determination of their hardness, thickness, mass, disintegration time, wettability, contact angle and swelling index values. The preformulation study confi rmed that characteristics of the active pharmaceutical ingredient might have modifi ed physicochemical properties of both powder formulations and tablets.