A drug administering device comprises: a casing front portion (2) in which a cavity (37) is formed and which comprises front connecting means (15), - a drug container (36) received in said cavity (37), - a casing rear portion comprising a piston rod which is displaceable into said cavity (37), and rear connecting means for releasably connecting to said front connecting means (15), wherein said casing front portion (2) comprises at least two members (12, 24) which, in locking engagement with each other, delimit a front portion (38) of said cavity (37) and a rear portion (39) having a cross section which is narrower than that of the front portion (38) and too narrow for the drug container (36) to pass.
