An injection device has a housing (34), and a carpule container (44) for receiving a carpule (50) having a fluid (52) to be injected and having a piston (48) displaceable in said carpule (50). It further has a piston rod (38) with end plate (46′) which serves, in the context of an injection, to displace the piston (48) of a carpule (50) inserted into the carpule container (44) and thereby to eject fluid (52) from the carpule (50). In order to minimize patient error in adjusting the amount of fluid medication (52) to be injected, the mechanism includes a first component (68) and a second component (92) which link (98) and interact with each other, to control when the carpule container (44) can rotate with respect to the housing (34), and to prevent creation of an axial gap (62) between end plate (46′) and piston (48).
