Abstract: An injection pen that is suitable for being carried in the clothes of a user-patient for the selective use thereof. The pen includes a housing having a cartridge having a variable content disposed therein. The cartridge has an outlet formed therein. A piston is disposed therein for altering the fluid content of the cartridge. A pump rod is biased against the piston for altering the fluid content of the cartridge. A drive mechanism is provided for moving the pump rod at a continuous rate. The electromotor is controlled by an electronic control unit including a memory means for storing data, such as that corresponding to the reservoir stock status and that corresponding to the number of units of injection fluid to be delivered to the user-patient. A comparator means is provided for comparing the number of units of fluid with the reservoir stock status for controlling the electromotor. Preferably, the control unit is an integrated circuit including a processor.