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.