A wearable automatic injection device (100, 600, 1800, 2400, 2800, 4100, 5100) free of a battery for providing a subcutaneous injection of a therapeutic agent into a patient, the wearable automatic injection device comprising: a housing (102, 202, 635, 735, 1802, 2002, 2102, 2402, 5102, 5204, 5302, 5404, 5504) comprising a patient contact portion securable to the patient; an injection assembly moveably disposed in the housing holding a hypodermic injection needle (118, 218, 625, 725, 1206, 1306, 1812, 2008, 5202, 5304) for insertion into the patient, the injection assembly moveable between a retracted position in which the injection needle does not protrude outside the housing and an extended position in which the injection needle protrudes outside the housing; a vessel (106, 206, 605, 705, 1102, 1202, 1302, 1400, 1606, 1706, 2004, 2506, 2804, 2902, 4104, 4602) provided in the housing for holding the therapeutic agent; a plunger (110, 210, 615, 715, 2408, 2508, 2802, 3916) moveably disposed in the vessel for ejecting the therapeutic agent from the vessel into the injection assembly; a plunger actuation mechanism (112, 212, 630, 730, 2406, 2510, 3900, 4000) for actuating the plunger using a mechanical or hydraulic mechanism within the vessel; a retraction trigger (5312, 5314) responsive to a change of state of the wearable automatic injection device from an injection state to a post-injection state, wherein the wearable automatic injection device enters the post-iniection state upon completion of delivery of a therapeutically effective dose of the therapeutic agent and enters the post-iniection state upon removal of the wearable automatic injection device from the patient prior to completion of delivery of the therapeutically effective dose of the therapeutic agent; and a retraction mechanism for automatically retracting the injection assembly from the extended position in the injection state to the retracted position in the post-injection state upon triggering by the retraction trigger. a housing (102, 202, 635, 735, 1802, 2002, 2102, 2402, 5102, 5204, 5302, 5404, 5504) comprising a patient contact portion securable to the patient; an injection assembly moveably disposed in the housing holding a hypodermic injection needle (118, 218, 625, 725, 1206, 1306, 1812, 2008, 5202, 5304) for insertion into the patient, the injection assembly moveable between a retracted position in which the injection needle does not protrude outside the housing and an extended position in which the injection needle protrudes outside the housing; a vessel (106, 206, 605, 705, 1102, 1202, 1302, 1400, 1606, 1706, 2004, 2506, 2804, 2902, 4104, 4602) provided in the housing for holding the therapeutic agent; a plunger (110, 210, 615, 715, 2408, 2508, 2802, 3916) moveably disposed in the vessel for ejecting the therapeutic agent from the vessel into the injection assembly; a plunger actuation mechanism (112, 212, 630, 730, 2406, 2510, 3900, 4000) for actuating the plunger using a mechanical or hydraulic mechanism within the vessel; a retraction trigger (5312, 5314) responsive to a change of state of the wearable automatic injection device from an injection state to a post-injection state, wherein the wearable automatic injection device enters the post-iniection state upon completion of delivery of a therapeutically effective dose of the therapeutic agent and enters the post-iniection state upon removal of the wearable automatic injection device from the patient prior to completion of delivery of the therapeutically effective dose of the therapeutic agent; and a retraction mechanism for automatically retracting the injection assembly from the extended position in the injection state to the retracted position in the post-injection state upon triggering by the retraction trigger.