A syringe unit (10) comprising: a cylinder (12) for containing one or more medications, a narrowed outlet (12a) to allow the expulsion of said one or more medications from said cylinder through said outlet； and a cap (14； 14 '； 14' ') disposed in the cylinder that defines and separates a first volume (24) and a second volume (26) inside the syringe unit, the cap comprising: a permanent seal ( 16) to the flow between the cap and the cylinder around the entire perimeter of the cap, between the first volume and the second volume； an elastic seal (22, 22 ')； and a channel (18) that evades the permanent seal, the channel that has a first opening (20a) in fluid communication with the second volume and a second opening (20b) selectively sealed with respect to the first volume by the elastic seal； wherein the elastic seal can move between a sealing configuration and an open configuration to selectively seal the channel with respect to the first volume, where, in the sealing configuration there is an elastic seal to the flow between the cap and the cylinder to isolate said second opening of the channel of the first volume, and in the open configuration the second opening of the channel is in fluid communication with the first volume； and wherein the elastic seal can move from the sealing configuration to the open configuration when the pressure of the fluid in the first volume or in the second volume exceeds a first pressure threshold.Una unidad de jeringa (10) que comprende: un cilindro (12) para contener uno o más medicamentos, una salida estrechada (12a) para permitir la expulsión de dichos uno o más medicamentos de dicho cilindro a través de dicha salida； y un tapón (14； 14'； 14'') dispuesto en el cilindro que define y separa un primer volumen (24) y un segundo volumen (26) dentro de la unidad de jeringa, el tapón que comprende: un sello permanente (16) al flujo entre el tapón y el cilindro alrededor de todo el perímetro del tapón, entre el primer volumen y el segundo vo
