An injector for injecting a liquid from a syringe with a movable syringe plunger. The injector includes a syringe receptacle for insertion of the syringe and, associated with the syringe receptacle, a drive unit for the syringe plunger. The drive unit is embodied as a spindle drive unit having a threaded spindle that can be driven around its longitudinal axis by a motor and having a spindle nut that is positioned on the threaded spindle, wherein the spindle nut is embodied as an unlocking slider with a through bore embodied as an oblong hole for the passage of the threaded spindle, whose maximum diameter extends perpendicular to the longitudinal axis of the threaded spindle and only in an end region of the maximum diameter, has a partial thread for the engagement in the threaded spindle, and the unlocking slider can be moved perpendicular to the longitudinal axis of the threaded spindle between an engagement position in which the partial thread engages with the threaded spindle and a release position in which the partial thread does not engage with the threaded spindle.
