An auto-injector device comprising an injector body having a proximal end and a distal end； a syringe received in the injector body； a needle disposed in a first end of the syringe to extend toward an opening in the distal end of the injector body； and a sealing element disposed at the distal end of the injector body for making a seal against a patient's skin. The auto-injector is configured to generate a reduced pressure to draw the auto-injector device against the patient's skin. The auto-injector device comprises a pressure-reducing mechanism configured to generate the reduced pressure within the injector body during an injection process. The pressure reducing mechanism comprises a moveable member configured such that movement of the moveable member from a first position to a second position enlarges a closed volume within the injector body to generate the reduced pressure within the closed volume.
