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.