Method for producing pharmaceutical drug particles of small particle size, characterised by the following steps: - Dispersing pharmaceutical drug particles in a water-miscible solvent - Pumping the thus-prepared dispersion under raised pressure through a dispersion conveyor line at the end of which a nozzle that functions as a pressure regulator is located - Dissolving the dispersed drug particles by heating the dispersion conveyor line to a temperature above the boiling point of the solvent at normal pressure - Passage of the drug solution through the nozzle of a precipitation/spray-drying reactor - Collision of the liquid jet of drug solution with a liquid jet formed by another nozzle of the precipitation/spray-drying reactor, the latter jet consisting of water or an aqueous solution optionally containing biocompatible constituents that modify the surface of the ultrafine particles formed during precipitation - Maintenance of a gaseous atmosphere at the collision point of the liquid jets by supplying gas to blow the precipitation zone free, or by at least partial vaporisation of solvent and water in the collision zone as a result of the pressure drop following the passage of the jets through the respective nozzles, or, where a free-jet reactor is used, i.e. a reactor with a sufficiently large collision zone, by gravity-based removal of the dispersion mist - Extremely rapid mixing due to mixing taking place in the form of impinging jets in a gaseous atmosphere, with a mixing time of less than 100 ms, preferably less than one ms - Formation of nanoparticulate nuclei by very fast diffusion-controlled solvent/non-solvent precipitation at the collision point and the plate-like mixing zone of the liquid jets in a gaseous atmosphere. - Dispersing pharmaceutical drug particles in a water-miscible solvent - Pumping the thus-prepared dispersion under raised pressure through a dispersion conveyor line at the end of which a nozzle that functions as a pressure regulator is located - Dissolving the dispersed drug particles by heating the dispersion conveyor line to a temperature above the boiling point of the solvent at normal pressure - Passage of the drug solution through the nozzle of a precipitation/spray-drying reactor - Collision of the liquid jet of drug solution with a liquid jet formed by another nozzle of the precipitation/spray-drying reactor, the latter jet consisting of water or an aqueous solution optionally containing biocompatible constituents that modify the surface of the ultrafine particles formed during precipitation - Maintenance of a gaseous atmosphere at the collision point of the liquid jets by supplying gas to blow the precipitation zone free, or by at least partial vaporisation of solvent and water in the collision zone as a result of the pressure drop following the passage of the jets through the respective nozzles, or, where a free-jet reactor is used, i.e. a reactor with a sufficiently large collision zone, by gravity-based removal of the dispersion mist - Extremely rapid mixing due to mixing taking place in the form of impinging jets in a gaseous atmosphere, with a mixing time of less than 100 ms, preferably less than one ms - Formation of nanoparticulate nuclei by very fast diffusion-controlled solvent/non-solvent precipitation at the collision point and the plate-like mixing zone of the liquid jets in a gaseous atmosphere.