Pharmaceutical drug particles of small particle size are produced by combining a solvent-non-solvent precipitate with an in-situ spray drying process. A pharmaceutical drug is dispersed in a water-miscible solvent, in particular ethanol, and heated in an infeed line under pressure to above the boiling point of the solvent until the drug dissolves. This solution collides as a fine liquid jet with a fine water jet in a gas-permeated microreactor, and the fine mist so arising vaporises rapidly. The organic solvent vaporizes first, then the water. The water may contain surface modifiers.