Disclosed is a transfilling device (1) for transfilling a liquid drug from a cartridge (3) to a flexible reservoir (4). The transfilling device (1) includes a support unit (111) and a plunger (113), the support unit (111) configured to receive the cartridge (3), the plunger (113) being insertable into the cartridge (3), wherein the plunger (113) is displaceable relative to the support unit (111) along an advancement direction (2) between a plunger retracted position, where the plunger (113) is in a most distal position relative to the support unit (111), and a plunger advanced position, where the plunger (113) is in a most proximal position relative to the support unit (111), wherein the plunger (113) is configured to push a piston (32) of the cartridge (3) from a piston retracted position to a piston advanced position by displacing the plunger (113). The transfilling device (1) further includes a transfer cannula (12, 12'), wherein the transfer cannula (12, 12') is configured to penetrate a flexible reservoir septum (41) with a cannula outlet end (122) and to penetrate a cartridge septum (31) with a cannula inlet end (121) and thereby to fluidly couple the flexible reservoir (4) with the cartridge (3). The transfilling device further includes a reservoir locking unit (13), wherein the reservoir locking unit () is configured to releasably mechanically couple the support unit (111) to the flexible reservoir (4) and wherein the coupling of the reservoir locking unit (13) is designed such that the coupling is released in the plunger advanced position.
