The present invention concerns a drive mechanism (3) for a drug delivery device (1), which comprises a piston rod (40) comprising a first piston rod member (7) and a second piston rod member (8). The drive mechanism (3) further comprises an adjustment member (9). The drive mechanism (3) has a first state in which the first and the second piston rod member (7, 8) are moveable with respect to each other by operating the adjustment member (9), thereby adjusting the length of the piston rod (40), and wherein the adjustment member (9) is arranged at least partially inside the second piston rod member (8). Moreover, the present invention concerns a method for assembling the drug delivery device (1).本發明係有關於藥物輸送裝置(1)之驅動機構(3)，其係包括：包含一第一活塞桿構件(7)及一第二活塞桿構件(8)的一活塞桿(40)。該驅動機構(3)更包括一調整構件(9)。該驅動機構(3)有一第一狀態，在處於第一狀態時，藉由操作該調整構件(9)，該第一及該第二活塞桿構件(7，8)可彼此相對移動，藉此調整該活塞桿(40)的長度，以及其中該調整構件(9)至少部份配置於該第二活塞桿構件(8)內。本發明也有關於組裝該藥物輸送裝置(1)之方法。
