The invention relates to an auto-injector (A) for administering a dose of a liquid medicament, comprising of a substantially tubular front-end device adapted to contain a syringe with an injection needle and a barrel containing the dose of the medicament and comprising a needle shroud adapted to rest on the skin of a patient receiving an injection and a reusable back-end device comprising of a housing, a plunger connected to or adapted to engage a stopper providing a fluid tight seal for a distal end of the barrel, a motor for displacing the plunger connected to the stopper, wherein the front-end device is attachable to the back-end device, wherein the needle shroud is slidably arranged with respect to the injection needle and wherein an interlock switch is capable of detecting an axial position (PA, PR) of the needle shroud.