A medical device for administering a medication to a patient, comprising: a syringe tube (24, 24a, 24b) that has a front end (124) and a rear end (224) and which defines a reservoir (25) within which the medicine may be contained； a needle cannula (26) having a front tip (126) and a rear end (226) and which is connected to a hub of the movable needle cannula (34) in said tube (24) between an initial position in which said front tip (126) is exposed and a retracted position in which said front tip (126) is contained within said tube (24), said rear end (226) of said needle cannula (26) being in fluid communication with said reservoir (25) through said needle cannula (26)； a plunger (28) having a first end with a plug (30, 30a, 30b) that can be placed in said reservoir (25) and a second end having a thumb tab (32) to receive pressure to administer the medicament to cause said plunger (28) to move inside said reservoir (25) to cause the medicament to be expelled from said reservoir (25)； a retainer (40, 40a, 40b) applied to an annular groove (38, 38a, 38b) defined on an inner surface of said tube (24, 24a, 24b) and releasably fixing said hub of the needle cannula (34, 34a, 34b) in said initial position close to the front end of said tube (24, 24a, 24b), and a driving member (42, 42a, 42b) disposed between said front end of said tube (24, 24a, 24b) and said needle cannula hub (34, 34a, 34b) to drive said needle cannula hub (34, 34a, 34b) with said needle cannula (26) to the retracted position in said tube (24), characterized in that: said retainer (40, 40a, 40b) is ring-shaped and has a cross-sectional shape with a radial projection outward (44, 44a, 44b) that is applied to the groove (38 , 38a, 38b) defined in said tube (24, 24a, 24b), a first leg (45, 45a, 45b) oriented towards said front end of said tube (24, 24a, 24b), and a second leg (46, 46a, 46b) orient ada towards said rear end of said tube (24, 24a, 24b)； and because said second leg (46, 46a, 46b) interacts wi
