The invention relates to an auto-injector (1) for administering a dose of a liquid medicament (M), comprising:- an elongate housing (2) arranged to contain a syringe (7) with a hollow needle (9) and a bung (10) for sealing the syringe (7) and displacing the medicament (M), the elongate housing (2) having a distal end (D) and a proximal end (P) with an orifice intended to be applied against an injection site, wherein the syringe (7) is slidably arranged with respect to the housing (2),- spring means (14) capable of, upon activation, pushing the needle (9) from a covered position inside the housing (2) into an advanced position through the orifice and past the proximal end (P) as well as operating the syringe (7) to supply the dose of medicament (M),- activating means (4) arranged to lock the spring means (14) in a pressurized state prior to manual operation and capable of, upon manual operation, releasing the spring means (14) for injection.The spring means (14) is a torsion spring (14) grounded at one end in the housing (2) and at the other end in a first gear member (15) rotatable about a longitudinal axis but axially fixed wherein the first gear member (15), upon rotation, is arranged for translatively moving a second gear member (11), the second gear member (11) prevented from rotating and coupled to the bung (10) in order to push it towards the proximal end (P), wherein the first gear member (15) is engaged with the activating means (4) prior to manual operation in a manner to prevent rotation and disengaged from the activating means (4) upon manual operation wherein the second gear member (11) is a piston rod (11) having an external lead screw thread (16), the piston rod (11) having an axial bore for slidably arranging the piston rod (11) on a shaft (21) attached to the housing (2), the axial bore and the shaft (21) having corresponding non-circular profiles, wherein the first gear member (15) is a lead nut (15) engaged with the external lead screw thread
