A drug delivery device comprising: a tubular body having a proximal end and a distal end, said tubular body defining an opening at or in proximity to said distal end, said opening defining an inner diameter; a drug cartridge having a proximal end, a distal end and a barrel portion disposed therebetween, said proximal end being open, at least one stopper being slideably disposed in said barrel portion, said distal end being sealed by a septum affixed to the barrel portion, the septum being piercable by a needle, said distal end of said drug cartridge being larger in diameter than said inner diameter of said opening in said tubular body, wherein, said drug cartridge is inserted into said tubular body to a use position by first inserting said proximal end of said drug cartridge into said opening in said tubular body, said distal end of said drug cartridge limiting insertion of said drug cartridge into said tubular body due to the interengagement of said distal end of said drug cartridge and said tubular body, and, wherein, with said drug cartridge being in said use position, said distal end of said drug cartridge is located at least partially externally of said tubular body, and, an adaptor engageable with said tubular body so as to encompass said distal end of said drug cartridge with said drug cartridge being in said use position and to secure said drug cartridge in said use position in said tubular body, said adaptor including features for mounting a needle assembly thereto, characterized in that said adaptor includes a reduced-diameter face defined therein, said reduced-diameter face defining a diameter smaller than the diameter of said distal end of said drug cartridge, said septum of said drug cartridge abuts said reduced-diameter face thereby limiting distal movement of said drug cartridge out of said tubular body with said drug cartridge being in said use position. a tubular body having a proximal end and a distal end, said tubular body defining an opening at or in proximity to said distal end, said opening defining an inner diameter; a drug cartridge having a proximal end, a distal end and a barrel portion disposed therebetween, said proximal end being open, at least one stopper being slideably disposed in said barrel portion, said distal end being sealed by a septum affixed to the barrel portion, the septum being piercable by a needle, said distal end of said drug cartridge being larger in diameter than said inner diameter of said opening in said tubular body, wherein, said drug cartridge is inserted into said tubular body to a use position by first inserting said proximal end of said drug cartridge into said opening in said tubular body, said distal end of said drug cartridge limiting insertion of said drug cartridge into said tubular body due to the interengagement of said distal end of said drug cartridge and said tubular body, and, wherein, with said drug cartridge being in said use position, said distal end of said drug cartridge is located at least partially externally of said tubular body, and, an adaptor engageable with said tubular body so as to encompass said distal end of said drug cartridge with said drug cartridge being in said use position and to secure said drug cartridge in said use position in said tubular body, said adaptor including features for mounting a needle assembly thereto, characterized in that said adaptor includes a reduced-diameter face defined therein, said reduced-diameter face defining a diameter smaller than the diameter of said distal end of said drug cartridge, said septum of said drug cartridge abuts said reduced-diameter face thereby limiting distal movement of said drug cartridge out of said tubular body with said drug cartridge being in said use position.