An injection device comprising: a housing; a needle hub, a needle mounted in said needle hub, said needle having a distal end piercing tip and a proximal end piercing tip, said needle hub being slidably mounted within said housing, said needle hub being moveable from a first position wherein said distal end piercing tip is within said housing to a second position wherein said distal end piercing tip extends outwardly of said housing; a spring biasing said needle hub towards said first position; a cartridge receiving member mounted within said housing; a plunger rod assembly including a plunger rod for exerting pressure on said cartridge receiving member to move a first cartridge mounted thereto and said needle hub from said first position to said second position; said plunger rod having a distal interior chamber and a proximal interior chamber, said distal interior chamber being designed to receive a second cartridge containing a liquid, said second cartridge having a plunger to prevent exit of said liquid; characterized in that an aperture provided in a side wall of said housing; said cartridge receiving member being accessible through said aperture in said side wall of said housing; said second cartridge has a septum; a second needle hub located in said proximal interior chamber, said second needle hub is slidable between a first non-piercing position and a second piercing position; a second hollow needle mounted in said second needle hub, said second needle having a distal piercing tip and a proximal piercing tip, said second hollow needle being moveable from a first non piercing position to a second position whereby said distal piercing tip will pierce said septum; and a second spring mounted so as to exert pressure on said plunger of said liquid containing cartridge. a housing; a needle hub, a needle mounted in said needle hub, said needle having a distal end piercing tip and a proximal end piercing tip, said needle hub being slidably mounted within said housing, said needle hub being moveable from a first position wherein said distal end piercing tip is within said housing to a second position wherein said distal end piercing tip extends outwardly of said housing; a spring biasing said needle hub towards said first position; a cartridge receiving member mounted within said housing; a plunger rod assembly including a plunger rod for exerting pressure on said cartridge receiving member to move a first cartridge mounted thereto and said needle hub from said first position to said second position; said plunger rod having a distal interior chamber and a proximal interior chamber, said distal interior chamber being designed to receive a second cartridge containing a liquid, said second cartridge having a plunger to prevent exit of said liquid; characterized in that an aperture provided in a side wall of said housing; said cartridge receiving member being accessible through said aperture in said side wall of said housing; said second cartridge has a septum; a second needle hub located in said proximal interior chamber, said second needle hub is slidable between a first non-piercing position and a second piercing position; a second hollow needle mounted in said second needle hub, said second needle having a distal piercing tip and a proximal piercing tip, said second hollow needle being moveable from a first non piercing position to a second position whereby said distal piercing tip will pierce said septum; and a second spring mounted so as to exert pressure on said plunger of said liquid containing cartridge.