A method provides for checking the correct coupling of an adding device to a therapeutic appliance that includes an extracorporeal circuit with which the adding device is connected such that a drug can be introduced into the extracorporeal circuit by the adding device during operation of the therapeutic appliance. The adding device is coupled to the extracorporeal circuit on the suction side of a pump disposed in the extracorporeal circuit, with a portion in the region of negative pressure of the extracorporeal circuit being in connection with a portion of the adding device that is closed with respect to the other parts of the extracorporeal circuit, and with the pressure in the closed portion being varied and measured.
