A medicament delivery device (1) comprises a housing (15), a telescopic piston rod assembly (12) for driving a bung (20) of a medicament container (9), and a drive mechanism (17) for the telescopic piston rod assembly. The telescopic piston rod assembly (12) has an input drive (23) telescopically coupled to a plunger (55) for driving the bung (20) via an intermediate coupling. The intermediate coupling comprises first and second cylinders (45, 49) telescopically coupled to one another. The plunger (55) is telescopically coupled to one of the first and second cylinders (45, 49) and the input drive (23) is telescopically coupled to the other cylinder. A key (14) is provided for restraining rotation of the plunger (55) relative to the housing (15) so that as the drive mechanism (17) drives the input drive, the plunger, first and second members and input drive telescopically expand or retract with respect to one another.
