The present invention relates to a handheld injection device comprising a housing (10, 30, 40； 40'), a piston rod (80) located within the housing (10, 30, 40； 40'), a drive member (70) permanently coupled to the piston rod (80), a dose setting member (50), which during dose setting rotates relative to the housing (10, 30, 40； 40') in a first direction and which during dose dispensing rotates relative to the housing (10, 30, 40； 40') in a second opposite direction, and a power reservoir (100) for driving the drive member (70). The drive member (70) is axially movable between a dose setting position, in which the drive member (70) is rotationally constrained to the housing, and a dose dispensing position, in which the drive member (70) is rotationally de-coupled from the housing (10, 30, 40； 40'). The power reservoir comprises a reverse wound flat spiral spring (100) having a first end attached to a first spool (101) and a second end attached to a second spool (102), with one of the spools (102) being coupled to the drive member (70).本發明係有關於一種手持注射裝置，其係包括：一殼體(10，30，40；40')，位於該殼體(10，30，40；40’)內的一活塞桿(80)，永久性耦合至該活塞桿(80)的一驅動構件(70)，在劑量設定期間對於該殼體(10，30，40；40')在第一方向旋轉以及在劑量分配期間對於該殼體(10，30，40；40')在相反第二方向旋轉的一劑量設定構件(50)，以及用於驅動該驅動構件(70)的一功率貯器(100)。該驅動構件(70)可在一劑量設定位置與一劑量分配位置之間軸向移動，該驅動構件(70)在該劑量設定位置時旋轉受拘束於該該殼體，以及該驅動構件(70)在該劑量分配位置時與該殼體(10，30，40；40')旋轉解耦。該功率貯器包括具有附接至第一捲軸(101)之第一末端以及附接至第二捲軸(102)之第二末端的一反繞平面渦卷彈簧(100)，以及該等捲軸(102)中之一者耦合至該驅動構件(70)。
