An auto-injector for delivering a liquid medicament comprising: a housing arranged to contain a syringe with a piston for sealing the syringe and displacing the medicament, the housing having a distal end and a proximal end intended to be applied against an injection site; a plunger unit arranged between the piston and the distal end of the housing, the plunger unit comprising a threaded internal surface for receiving and engaging a threaded rod element; and a tensioned elastic element fixed to the threaded rod element, the elastic element and the threaded rod element being fixed against rotation in an initial state, wherein when the elastic element is rotationally released, the threaded rod element rotates in a first direction, causing the plunger unit to translate towards the proximal end of the housing and thereby pushing the piston towards the proximal end of the housing to displace the medicament.