Syringe with syringe barrel, syringe head and ejector unit

A syringe (20) comprising a syringe barrel (15) for holding a medicine to be administered, an ejector unit (25) arranged at a distal end of the syringe barrel (15), a syringe head (21) arranged at a proximal end of the syringe barrel (15) with a needle unit (12) arranged in a guide sleeve (5) and slidable therein comprising a cannula (3) and a cannula holder (4) and a pinion (2), by means of which the needle unit (12) can be moved axially relative to a syringe barrel (15) from a position of rest to an actuation position and a sealing element (7) arranged at a distal end of the guide sleeve (5) which in the position of rest of the needle unit (12) is unpierced by the cannula (3) and in an activated position of the needle unit (12) is pierced by the cannula (3), wherein the pinion (2) comprises two guide knobs (1), each of which can be moved into engagement optionally with one of two slide tracks (10) provided in the guide sleeve (5), wherein the slide tracks (10) are inclined in opposite directions, and wherein the two guide knobs (1) are arranged spaced apart from one another in a plane aligned perpendicular to a longitudinal axis and in a circumferential direction and in order to displace the needle unit (12) from the position of rest to the actuation position only one of the two guide knobs (1) is moved into engagement with one of the two slide tracks (10).