A pre-filled injection device includes a tubular containment body extending between a first and second openings, inside which are hermetically separated first and second containment chambers. A first substance is contained in the first containment chamber. First and second plugs inside the tubular body define the second containment chamber, and slide inside the tubular body. A second liquid substance inside the second containment chamber is intended to be mixed inside the tubular body with the first substance for reconstituting an injectable solution. A dosing reservoir has an inlet and an outlet, the inlet being in communication with the first containment chamber so that the first containment chamber extends between the first plug and the dosing reservoir inlet. The dosing reservoir contains a fraction of the injectable solution. The first plug includes a projecting appendix passing through and entering the dosing reservoir to eject the fraction of solution.
