The present invention concerns a prefilled syringe (400) with the typical tubular containment body (430), which extends along an axis between a front end element (420) and a rear end element (410), and wherein a fixed plug (470) and two sliding plugs (460, 450) are arranged. There is a bypass channel in the initial position of the syringe (400) between the first two plugs (470, 460). The two plugs (470,— 460) form, together with a third plug (450) at the end of the syringe plunger, a first containment chamber (400L1) containing a first liquid substance and a second containment chamber (400L2) containing a second solid or liquid substance. In the prefilled syringe (400), the first plug (470): is arranged in a fixed position in contact with said front end element (420)； is provided with a non-through cavity (445) offset with respect to said axis and facing towards said front end element (420).
