A preloaded injection device (1), comprising: - a tubular containment body (3), which extends between a first opening (4) and a second opening (5), inside which a first ( 8) and a second (9) containment chambers, which are hermetically separated from each other； - a first solid or liquid substance, contained in the first containment chamber (8)； - a first (6) and a second (7) plug, arranged inside the tubular body (3), to define between them in the tubular body (3) the second containment chamber (9) and adapted to slide in the inside the tubular body (3)； - a second liquid substance, contained inside the second containment chamber (9), intended to be mixed inside the tubular body (3) with the first substance for reconstitution of an injectable solution； characterized in that: - the injection device (1) further comprises a dosing tank (20), which has an inlet opening (21) and an outlet opening (22), the inlet opening (21) being in communication with the first containment chamber (8) so that the first containment chamber (8) extends between the first stopper (6) and the inlet opening (21) of the dosing tank (20)； - the dosing tank (20) is adapted to contain a fraction of said injectable solution； - the first cap (6) comprises an protruding appendix (16) adapted to pass through the inlet opening (21) of the dosing tank (20) and enter the latter to expel said fraction of solution from the tank (20) of dosage.Un dispositivo (1) de inyección precargado, que comprende: - un cuerpo (3) de contención tubular, que se extiende entre una primera abertura (4) y una segunda abertura (5), en el interior del cual se definen una primera (8) y una segunda (9) cámaras de contención, que están separadas herméticamente entre sí； - una primera sustancia sólida o líquida, contenida en la primera cámara (8) de contención； - un primer (6) y un segundo (7) tapón, dispuestos en el interior del cuerpo (3) tubular, para definir entre ellos en el cuerpo (3) tubular la segunda cámara (9) de conte
