A wearable automatic injection device free of a battery for administering a therapeutic agent to a patient, the wearable automatic injection device comprising: a housing (102, 202, 635, 735, 1102, 1202, 1302, 1402, 3102, 3204, 3302, 3404, 3504) comprising a patient contact portion including a retraction trigger configured to contact the patient, the retraction trigger responsive to a change of state of the wearable automatic injection device from an administration state to a post-administration state; an administration assembly moveably disposed in the housing holding an administration interface (118, 218, 625, 725, 1112, 1308, 1606, 1706, 3202) configured to administer the therapeutic agent to the patient, the administration assembly moveable between a retracted position in which the administration interface (118, 218, 625, 725, 1112, 1308, 1606, 1706, 3202) does not protrude outside the housing and an extended position in which the administration interface (118, 218, 625, 725, 1112, 1308, 1606, 1706, 3202) protrudes outside the housing (102, 202, 635, 735, 1102, 1202, 1302, 1402, 3102, 3204, 3302, 3404, 3504); a vessel (106, 206, 605, 705, 1602, 1702, 1800, 2006, 2106, 2404, 2602) provided in the housing (102, 202, 635, 735, 1102, 1202, 1302, 1402, 3102, 3204, 3302, 3404, 3504) configured to hold the therapeutic agent (108, 208); a plunger actuation mechanism (112, 212, 630, 730) moveably disposed in the vessel (106, 206, 605, 705, 1602, 1702, 1800, 2006, 2106, 2404, 2602) configured to eject the therapeutic agent (108, 208) from the vessel (106, 206, 605, 705, 1602, 1702, 1800, 2006, 2106, 2404, 2602) into the administration assembly; and a retraction mechanism configured to automatically retract the administration assembly from the extended position in the administration state to the retracted position in the post-administration state upon triggering by the retraction trigger; wherein a volume of the therapeutic agent of between 0.1 milliliters and 1.0 milliliter is ejected from the vessel in less than thirty seconds. a housing (102, 202, 635, 735, 1102, 1202, 1302, 1402, 3102, 3204, 3302, 3404, 3504) comprising a patient contact portion including a retraction trigger configured to contact the patient, the retraction trigger responsive to a change of state of the wearable automatic injection device from an administration state to a post-administration state; an administration assembly moveably disposed in the housing holding an administration interface (118, 218, 625, 725, 1112, 1308, 1606, 1706, 3202) configured to administer the therapeutic agent to the patient, the administration assembly moveable between a retracted position in which the administration interface (118, 218, 625, 725, 1112, 1308, 1606, 1706, 3202) does not protrude outside the housing and an extended position in which the administration interface (118, 218, 625, 725, 1112, 1308, 1606, 1706, 3202) protrudes outside the housing (102, 202, 635, 735, 1102, 1202, 1302, 1402, 3102, 3204, 3302, 3404, 3504); a vessel (106, 206, 605, 705, 1602, 1702, 1800, 2006, 2106, 2404, 2602) provided in the housing (102, 202, 635, 735, 1102, 1202, 1302, 1402, 3102, 3204, 3302, 3404, 3504) configured to hold the therapeutic agent (108, 208); a plunger actuation mechanism (112, 212, 630, 730) moveably disposed in the vessel (106, 206, 605, 705, 1602, 1702, 1800, 2006, 2106, 2404, 2602) configured to eject the therapeutic agent (108, 208) from the vessel (106, 206, 605, 705, 1602, 1702, 1800, 2006, 2106, 2404, 2602) into the administration assembly; and a retraction mechanism configured to automatically retract the administration assembly from the extended position in the administration state to the retracted position in the post-administration state upon triggering by the retraction trigger;