Small crystals are made by mixing a solution of a desired substance with an anti-solvent in a fluidic vortex mixer in which the residence time is less than 1s, for example 10 ms. The liquid within the fluidic vortex mixer (12) is subjected to high intensity ultrasound from a transducer (20, 22). The solution very rapidly becomes supersaturated, and the ultrasound can induce a very large number of nuclei for crystal growth. Small crystals, for example less than 5 μm, are formed. The resulting suspension is treated so as to add or remove ingredients, and then spray dried using an atomiser tuned to create small droplets in such a way that each droplet should contain not more than one crystal. Crystal agglomeration is hence prevented.