An auto-injector (1) comprising: a housing (2) adapted to hold a medicament container having a needle (4)； a needle cover (7) telescopically coupled to the housing (2) and which can move between a first extended position (FEP) with respect to the housing (2) in which the needle (4) is covered and a position retracted (RP) with respect to the housing (2) in which the needle (4) is exposed； and a piston (10) rotatably and slidably arranged in the housing (2), the piston (10) being able to rotate with respect to the housing (2) between a first rotation position in which the piston (10) is engaged to the housing (2) and a second rotation position in which the plunger (10) is disengaged from the housing (2), in which the needle cover (7) is operatively coupled to the plunger (10), and wherein, when the needle cover (7) moves from the first extended position (FEP) to the retracted position (RP), the piston (10) rotates from the first rotation position to the second rotation position, characterized in that the piston (10) includes a first piston projection (10.1) adapted to engage a cover rib (7.7) disposed on the needle cover (7) and a second piston projection (10.2) adapted to engage a a housing groove (2.3) in the housing (2), in which, when the plunger (1 0) is in the first rotation position and the needle cover (7) is in the first extended position (FEP), the first piston projection (10.1) engages the cover rib (7.7) and the second piston projection (10.2) is attached to the housing groove (2.3), in which, when the needle cover (7) is moved from the first extended position (FEP) to the retracted position (RP), the piston (10) rotates from the first rotation position to the second rotation position and the second piston projection (10.2) is disengaged from the housing groove (2.3).Un autoinyector (1) que comprende: una carcasa (2) adaptada para sujetar un contenedor de medicamento que tiene una aguja (4)； una cubierta de aguja (7) acoplada de manera telescópica a la ca
