It is described a pre-filled injection device (1), comprising : - a tubular containment body (3), extending between a first opening (4) and a second opening (5), inside which a first (8) and a second (9) containment chamber are defined which are hermetically separated from one another； - a first solid or liquid substance, contained in the first containment chamber (8)； - a first (6) and a second (7) plugs, arranged inside the tubular body (3), so as to define therebetween in the tubular body (3) the second containment chamber (9), and adapted to slide 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 the reconstitution of an injectable solution. The injection device further comprises a dosing reservoir (20), having 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 plug (6) and the inlet opening (21) of the dosing reservoir (20). The dosing reservoir (20) is adapted to contain a fraction of the injectable solution. The first plug (6) comprises a projecting appendix (16) adapted to pass through the inlet opening (21) of the dosing reservoir (20) and to enter the latter in order to eject the fraction of solution from the dosing reservoir (20).
