An automatic injection device 101 has a mechanical interlock preventing actuation of its trigger 105 until removal of its needle boot 111. The interlock may be between a boot remover 104 (such as a cap) and a trigger cover 106, the cover having flexible ridges 109 which are compressed by the boot remover to prevent removal (figure 1). Alternatively, the injection device may comprise a separate syringe housing (203, figure 4) and a firing mechanism housing (202). When the two are joined together a lip (205) on the firing mechanism housing axially displaces a rod (207) residing in the syringe housing which in turn displaces the boot remover (204). Alternatively, a helical track in the rod may engage with a peg on the boot remover to rotate the rod and thereby deactivate a trigger lock (figures 8-13) such as a rotatable arm (405, figure 16). The two housings may be hinged together and held in a folded position by the boot remover 502 such that the device cannot operate until the boot remover is removed.
