A reusable medication delivery device for delivering medication from a cartridge having a barrel holding the medication between a movable plunger and an outlet, the reusable medication delivery device comprising: a main housing; a cartridge housing adapted to removably receive the cartridge, said cartridge housing detachably mountable to said main housing; a drive member including a forward end for engaging the movable plunger, said drive member having a length extending in an axial direction within the main housing and including a sensible element along a portion of said length; a dose delivery mechanism for controlling advancement of said drive member forward within the main housing in the axial direction from a rearward position to a forward position, wherein advancement of said drive member forward in the axial direction, when the cartridge housing receives the cartridge and is mounted to the main housing, moves the movable plunger for delivering medication through the outlet; means for determining an amount of medication delivered by operation of said dose delivery mechanism; a first sensor for sensing a presence of the cartridge within said cartridge housing when said cartridge housing is mounted to said main housing; at least one second sensor for sensing said sensible element as being located at a position within said main housing which is associated with said drive member being retracted to at least a first retraction position axially rearward of said forward position, wherein said first retraction position is no more than a first amount axially forward of a position associated with an initial engagement of the movable plunger of a full said cartridge, said first amount equal to or less than one-half of a total possible travel distance of said drive member relative to said main housing; and a controller in communication with said first sensor and said at least one second sensor and programmed to determine that a newly installed cartridge is to be considered full of medication when an input from said first sensor changes from a signal associated with no cartridge sensed as present to a signal associated with a cartridge being sensed as present and an input from said at least one second sensor is associated with said drive member being in a position at which said sensible element is aligned with or rearward of the second sensor, said controller in communication with said determining means and programmed to calculate, based on an input from said determining means after a medication delivery, and if the cartridge was considered full when newly installed, a quantity of medication remaining in the installed cartridge after the medication delivery. a main housing; a cartridge housing adapted to removably receive the cartridge, said cartridge housing detachably mountable to said main housing; a drive member including a forward end for engaging the movable plunger, said drive member having a length extending in an axial direction within the main housing and including a sensible element along a portion of said length; a dose delivery mechanism for controlling advancement of said drive member forward within the main housing in the axial direction from a rearward position to a forward position, wherein advancement of said drive member forward in the axial direction, when the cartridge housing receives the cartridge and is mounted to the main housing, moves the movable plunger for delivering medication through the outlet; means for determining an amount of medication delivered by operation of said dose delivery mechanism; a first sensor for sensing a presence of the cartridge within said cartridge housing when said cartridge housing is mounted to said main housing; at least one second sensor for sensing said sensible element as being located at a position within said main housing which is associated with said drive member being retracted to at least a first retraction position axially rearward of said forward position, wherein said first retraction position is no more than a first amount axially forward of a position associated with an initial engagement of the movable plunger of a full said cartridge, said first amount equal to or less than one-half of a total possible travel distance of said drive member relative to said main housing; and a controller in communication with said first sensor and said at least one second sensor and programmed to determine that a newly installed cartridge is to be considered full of medication when an input from said first sensor changes from a signal associated with no cartridge sensed as present to a signal associated with a cartridge being sensed as present and an input from said at least one second sensor is associated with said drive member being in a position at which said sensible element is aligned with or rearward of the second sensor, said controller in communication with said determining means and programmed to calculate, based on an input from said determining means after a medication delivery, and if the cartridge was considered full when newly installed, a quantity of medication remaining in the installed cartridge after the medication delivery.