Pharmaceutics manufacturers tend to use a same type of drug container for different drugs or for different preparations of a same drug due to the high costs of other methods. This can cause a serious problem if a patient uses inadvertently replaces an empty drug container by a container of a different drug or a different preparation. The present invention overcomes this problem by providing a drug administering device which includes a casing front portion in which a cavity is formed comprising a front connecting means a drug container in the cavity and a casing rear portion which comprises a piston rod displaceable into the cavity and a rear connecting means to releasably connect to the front connecting means. The casing front portion comprises at least two members which, in locking engagement with each other, delimit a front portion of the cavity and a rear portion having a cross section which is narrower than that of the front portion and too narrow for the drug container to pass. Hence a drug container which has been placed in the front portion of the cavity by the drug manufacturer cannot be removed by the patient in a way which would allow the casing front portion to be re-used with another drug container.
