A method of using a fluid dispenser unit having a reservoir and a piston that slides in the reservoir. The unit includes an extractor head secured to the reservoir to suck fluid to be dispensed into the reservoir, and a spray head assembled on the extractor head to spray the fluid out from the reservoir. The method includes the steps of sucking a fluid to be dispensed into the reservoir through the extractor head； assembling the spray head on the extractor head； and spraying the fluid through the spray head. The step of sucking up a fluid to be dispensed includes a first step of extracting a fluid, such as a solvent, from an extraction reservoir, and a second step of mixing the fluid with a first substance, such as a powder or a lyophilisate, so as to form the fluid to be dispensed.
