A medication injector for injecting for administering multiple doses of a medication comprising: a housing having a longitudinal axis and having a proximal end and a distal end and further including an internal viewing portion comprising a cut-out window for viewing inside the housing located at the distal end of the housing and a dose selection knob at the proximal end; a medication cartridge comprising an ampoule or syringe having a stopper at the proximal end and containing a medication positioned inside the distal end of the housing and; a piston rod in the housing that is oriented in the direction of the longitudinal axis and is engaged with a drive mechanism that advances the piston rod axially downward with rotation during dose delivery wherein the distal end of the piston rod pushes against the stopper in the medication cartridge, which drives the stopper distally in the medication cartridge to dispense the medication; and an alert indicator on or carried by the piston rod for signaling that the injector is approaching the end of its medication supply, the alert indicator having a hidden position in which is generally not visible through the internal viewing portion and a visible position in which it is viewable through the internal viewing portion, the alert indicator moving from the hidden position to the visible position when the piston rod is advanced distally a predetermined distance, characterized in that (i) the piston rod has an upper threading portion and a lower threading portion that is located distal to the upper threading portion, the upper threading portion having a thread form that is different than the thread form on the lower threading portion; and the alert indicator is (i) a signal nut or disc having a threaded hole through which the piston rod is inserted, the signal nut or disc mating with and being rotatable on the lower threading portion of the piston rod, but interfering with and not rotatable on the upper threading portion of the piston rod wherein the signal nut or disc has a different color than an external color of the housing around the internal viewing portion. a housing having a longitudinal axis and having a proximal end and a distal end and further including an internal viewing portion comprising a cut-out window for viewing inside the housing located at the distal end of the housing and a dose selection knob at the proximal end; a medication cartridge comprising an ampoule or syringe having a stopper at the proximal end and containing a medication positioned inside the distal end of the housing and; a piston rod in the housing that is oriented in the direction of the longitudinal axis and is engaged with a drive mechanism that advances the piston rod axially downward with rotation during dose delivery wherein the distal end of the piston rod pushes against the stopper in the medication cartridge, which drives the stopper distally in the medication cartridge to dispense the medication; and an alert indicator on or carried by the piston rod for signaling that the injector is approaching the end of its medication supply, the alert indicator having a hidden position in which is generally not visible through the internal viewing portion and a visible position in which it is viewable through the internal viewing portion, the alert indicator moving from the hidden position to the visible position when the piston rod is advanced distally a predetermined distance, characterized in that (i) the piston rod has an upper threading portion and a lower threading portion that is located distal to the upper threading portion, the upper threading portion having a thread form that is different than the thread form on the lower threading portion; and the alert indicator is (i) a signal nut or disc having a threaded hole through which the piston rod is inserted, the signal nut or disc mating with and being rotatable on the lower threading portion of the piston rod, but interfering with and not rotatable on the upper threading portion of the piston rod wherein the signal nut or disc has a different color than an external color of the housing around the internal viewing portion. (i) the piston rod has an upper threading portion and a lower threading portion that is located distal to the upper threading portion, the upper threading portion having a thread form that is different than the thread form on the lower threading portion; and the alert indicator is (i) a signal nut or disc having a threaded hole through which the piston rod is inserted, the signal nut or disc mating with and being rotatable on the lower threading portion of the piston rod, but interfering with and not rotatable on the upper threading portion of the piston rod wherein the signal nut or disc has a different color than an external color of the housing around the internal viewing portion.