A method for filling a cylinder/piston unit (1) for a needle-free injector comprising a chamber (3) arranged in a cylinder (2), for receiving an injection solution (4), a front wall (5) having at least one nozzle borehole or an outlet element (6), and a second piston (7) arranged to move in the chamber. The chamber (3) is made in two parts with a first chamber (8) and a concentric second chamber (9) having a smaller cross section than the first chamber. A first piston (10) closes the first chamber (8) and the second piston closes the second chamber (9). The second piston comprises at least one flow channel (16) or the first chamber comprises at least one closable, axially oriented freeze-drying channel (34), and the second chamber (9) comprises at least one axially oriented flow channel (36), on the respective rear ends of the chambers (8, 9).