The invention relates to a method for using an assembly for dispensing a fluid product including a vessel (10) intended for containing a fluid product and a piston (20) sliding inside said vessel (10), said assembly including a collection head (30) rigidly connected to said vessel (10) so as to suck a fluid product to be dispensed into said vessel (10), and a spray head (40) assembled on said collection head (30) so as to spray said fluid product to be dispensed out of said vessel (10), said method including the following steps: sucking a fluid product to be dispensed into said vessel (10) through said collection head (30) assembling said spraying head (40) on said collection head (30) and spraying said fluid product to be dispensed through said spraying head (40), characterised in that said step of sucking a fluid product to be dispensed includes a first step of collecting a fluid, such as a solvent, from a collection vessel (1), and a second step of mixing said fluid with a first product, such as a powder or a lyophilizate, so as to form said fluid product to be dispensed.
