The invention relates to an auto-injector (A) for administering a dose of a liquid medicament, comprising: - a substantially tubular front-end device (1) adapted to contain a syringe (2) with an injection needle (2.2) and a barrel (2.3) containing the dose of the medicament and comprising a needle shroud (1.4) adapted to rest on the skin of a patient receiving an injection and - a reusable back-end device (3) comprising - a housing (3.1), - a plunger (3.2) connected to or adapted to engage a stopper (3.3) providing a fluid tight seal for a distal end of the barrel (2.3), - a motor (3.5) for displacing the plunger (3.2) connected to the stopper (3.3), wherein the front-end device (1) is attachable to the back-end device (3), wherein the needle shroud (1.4) is slidably arranged with respect to the injection needle (2.2) and wherein an interlock switch (3.10) is capable of detecting an axial position (PA, PR) of the needle shroud (1.4).
