An autoinjector having a central axis within a housing, comprising: a power spring disposed at a proximal end of the autoinjector; a center screw ; a middle screw ; wherein the middle screw is an outer part of a double-acting screw and the center screw is an inner part of the double-acting screw; a syringe disposed along the central axis and comprising a plunger and a tip or needle hub disposed distally of the center screw so that movement of the center screw pushes the plunger ; and further characterized by a drive nut connected at the distal end to the power spring , wherein the drive nut comprises keys that drive the middle screw ; the middle screw cylindrically disposed about the drive nut ; wherein threads of an inner diameter of the middle screw and outer diameter of the center screw interact so that rotation of the middle screw , when driven by the drive nut , causes the center screw to advance in the distal direction; a retract screw cylindrically disposed about the middle screw and having threads on the outer circumference that match with threads on the inner circumference of the housing such that rotation of the middle screw , which comprises threads on its outer diameter that engage the retract screw , moves the retract screw in the proximal direction. a power spring disposed at a proximal end of the autoinjector; a center screw ; a middle screw ; wherein the middle screw is an outer part of a double-acting screw and the center screw is an inner part of the double-acting screw; a syringe disposed along the central axis and comprising a plunger and a tip or needle hub disposed distally of the center screw so that movement of the center screw pushes the plunger ; and further characterized by a drive nut connected at the distal end to the power spring , wherein the drive nut comprises keys that drive the middle screw ; the middle screw cylindrically disposed about the drive nut ; wherein threads of an inner diameter of the middle screw and outer diameter of the center screw interact so that rotation of the middle screw , when driven by the drive nut , causes the center screw to advance in the distal direction; a retract screw cylindrically disposed about the middle screw and having threads on the outer circumference that match with threads on the inner circumference of the housing such that rotation of the middle screw , which comprises threads on its outer diameter that engage the retract screw , moves the retract screw in the proximal direction. a drive nut connected at the distal end to the power spring , wherein the drive nut comprises keys that drive the middle screw ; the middle screw cylindrically disposed about the drive nut ; wherein threads of an inner diameter of the middle screw and outer diameter of the center screw interact so that rotation of the middle screw , when driven by the drive nut , causes the center screw to advance in the distal direction; a retract screw cylindrically disposed about the middle screw and having threads on the outer circumference that match with threads on the inner circumference of the housing such that rotation of the middle screw , which comprises threads on its outer diameter that engage the retract screw , moves the retract screw in the proximal direction. <EOS>