A prefilled injection device , including: - a tubular containment body , extending between a first opening and a second opening , inside which a first and second containment chamber are defined which are hermetically divided from each other; - a first liquid substance, being contained in the first containment chamber ; - a first and second plug being arranged inside the tubular body , in order to delimitate the first containment chamber therebetween in the tubular body , and suitable to slide inside the tubular body ; - a second solid or liquid substance, being contained in the second containment chamber , intended to be mixed with the first substance inside the tubular body in order to reconstruct an injectable solution; - at least one bypass channel, being initially in a closed state and suitable to be brought in a opened state in order to reconstruct the injectable solution; - a closing element being fastened to the tubular body ; - a front plug being arranged inside the tubular body after the second plug in order to delimitate the second containment chamber therewith in the tubular body , and suitable to slide inside the tubular body ; - a front chamber , being hermetically divided from the second containment chamber , delimited on one side by the front plug and on the opposite side by the closing element , said front chamber being empty so that the closing element is divided from the second substance; characterized in that : - the front plug or the second plug is provided with at least one stud protruding from the side facing the second chamber ; - the tubular body includes an internal wall being provided with a recess adapted to define the bypass channel, and wherein, in an initial configuration, the bypass channel is arranged between the front plug and the first opening . - a tubular containment body , extending between a first opening and a second opening , inside which a first and second containment chamber are defined which are hermetically divided from each other; - a first liquid substance, being contained in the first containment chamber ; - a first and second plug being arranged inside the tubular body , in order to delimitate the first containment chamber therebetween in the tubular body , and suitable to slide inside the tubular body ; - a second solid or liquid substance, being contained in the second containment chamber , intended to be mixed with the first substance inside the tubular body in order to reconstruct an injectable solution; - at least one bypass channel, being initially in a closed state and suitable to be brought in a opened state in order to reconstruct the injectable solution; - a closing element being fastened to the tubular body ; - a front plug being arranged inside the tubular body after the second plug in order to delimitate the second containment chamber therewith in the tubular body , and suitable to slide inside the tubular body ; - a front chamber , being hermetically divided from the second containment chamber , delimited on one side by the front plug and on the opposite side by the closing element , said front chamber being empty so that the closing element is divided from the second substance; - the front plug or the second plug is provided with at least one stud protruding from the side facing the second chamber ; - the tubular body includes an internal wall being provided with a recess adapted to define the bypass channel, and wherein, in an initial configuration, the bypass channel is arranged between the front plug and the first opening . <EOS>