A medication injection or delivery system is provided for injecting fluids such as insulin within body tissue. The device includes a housing, a flexible shaft movably mounted within the housing, and a dose setting mechanism for controlling the movement of the shaft with respect to a fluid-containing cartridge removably positioned within or mounted to the housing. The flexible shaft, according to exemplary embodiments, includes a plunger and a flexible cable connected to the dose setting mechanism. The plunger and dose setting mechanism are connected together by the flexible shaft. The dose setting mechanism, in one aspect of the invention, includes a screw, a locking mechanism, and a nut assembly connected to the flexible shaft. The dose setting mechanism is used to set the dosage and prime the catheter. The plunger retracts into a recess to permit a new cartridge to be installed quickly.