A pen-type drug delivery device, comprising: - a dose setting member; - a cartridge holder (104, 740), wherein said cartridge holder (104, 740) is secured to said dose setting member; - a cartridge including a generally tubular barrel extending from a distal end to a proximal end, the distal end being defined by an inwardly converging shoulder, a sleeve or ferrule located at the distal end, a smaller diameter neck projecting distally from the shoulder of the barrel and a pierceable seal or septum securely mounted across the open distal end defined by the neck and held in place by the sleeve or ferrule, - a locking mechanism (200, 700) comprising a main body, - the locking mechanism (200, 700) attaching the cartridge within the cartridge holder (104, 740), - the main body having an outer diameter and a centrally located aperture, which extends from a proximal end to a distal end of the main body, - at least one bendable feature (230, 930) provided on the main body, the bendable feature (230, 930) comprising a first section (232, 932), a second section (234, 934), and a third section (236, 936), the first section (232, 932) and the third section (236, 936) being affixed to the main body, - the bendable feature (230, 930) being movable from a first position, in which at least a portion of the second section (234, 934) of the bendable feature (230, 930) extends beyond the outer diameter of the main body, to a second position, the portion moving toward aperture until the bendable feature (230, 930) snaps into the second position, in said second position, at least a portion of said bendable feature (230, 930) being provided to retain said cartridge (120, 500, 750) within said body when the aperture is placed over the sleeve or ferrule, characterized in that - the bendable feature (230, 930) deforms with the second section (234, 934) moving toward the aperture until the bendable feature (230, 930) snaps into an inverted position, and - a non-return feature is provided on the main body, the bendable feature (230, 930) moving from the first position past the non-return feature to the second position and being held in the second position by the non-return feature. - a dose setting member; - a cartridge holder (104, 740), wherein said cartridge holder (104, 740) is secured to said dose setting member; - a cartridge including a generally tubular barrel extending from a distal end to a proximal end, the distal end being defined by an inwardly converging shoulder, a sleeve or ferrule located at the distal end, a smaller diameter neck projecting distally from the shoulder of the barrel and a pierceable seal or septum securely mounted across the open distal end defined by the neck and held in place by the sleeve or ferrule, - a locking mechanism (200, 700) comprising a main body, - the locking mechanism (200, 700) attaching the cartridge within the cartridge holder (104, 740), - the main body having an outer diameter and a centrally located aperture, which extends from a proximal end to a distal end of the main body, - at least one bendable feature (230, 930) provided on the main body, the bendable feature (230, 930) comprising a first section (232, 932), a second section (234, 934), and a third section (236, 936), the first section (232, 932) and the third section (236, 936) being affixed to the main body, - the bendable feature (230, 930) being movable from a first position, in which at least a portion of the second section (234, 934) of the bendable feature (230, 930) extends beyond the outer diameter of the main body, to a second position, the portion moving toward aperture until the bendable feature (230, 930) snaps into the second position, in said second position, at least a portion of said bendable feature (230, 930) being provided to retain said cartridge (120, 500, 750) within said body when the aperture is placed over the sleeve or ferrule, characterized in that - the bendable feature (230, 930) deforms with the second section (234, 934) moving toward the aperture until the bendable feature (230, 930) snaps into an inverted position, and - a non-return feature is provided on the main body, the bendable feature (230, 930) moving from the first position past the non-return feature to the second position and being held in the second position by the non-return feature.