The invention relates to an auto-injector (1) for administering a dose of a liquid medicament (M), comprising: - an elongate housing (2), a syringe (7) with a hollow needle (9) and a bung (10) for displacing the medicament (M), wherein the syringe (7) is slidably arranged with respect to the housing (2), - spring means (14) for pushing the needle (9) from a covered position inside the housing (2) into an advanced position past the proximal end (P) and for supplying the dose of medicament (M), - activating means (4) for locking and releasing the spring means (14). The spring means (14) is a torsion spring (14) grounded at one end in the housing (2) and at the other end in a lead nut (15) rotatable about a longitudinal axis but axially constrainable. The lead nut (15) is arranged for translatively moving a piston rod (11) by means of a lead screw thread (16). The piston rod (11) is prevented from rotating relative to the housing and arranged to be coupled to the bung (10) in order to push it forwards. The lead nut (15) is engaged to the housing (2) in an initial position prior to manual operation of the activating means (4) in a manner to prevent rotation and disengaged from the housing (2) by the activating means (4) upon manual operation. The activating means is a trigger button (4), arranged at a distal end (D) of the housing (2). Prior to manual operation the trigger button (4), the piston rod (11) and the lead nut (15) are coupled for joint translation in proximal direction (P). In the initial position the lead nut (15) is engaged to the housing (2) by at least one spline feature (41). The lead nut (15) is arranged to disengage from the housing (2) on translation in proximal direction (P) from the initial position.Linvention concerne un injecteur automatique (1) pour administrer une dose dun médicament liquide (M), comprenant :- un boîtier allongé (2), une seringue (7) avec une aiguille creuse (9) et une bonde (10) pour déplacer le médicament (M), la seringue (7)
