A supplemental device for attachment to an injection device, the supplemental device comprising: a housing； an electromechanical switch arrangement having an open state and a closed state , the electromechanical switch arrangement comprising a protrusion configured to contact a surface of the injection device while the supplemental device is attached to the injection device； a dose dialled detector operable to detect a dose of medicament dialled into the attached injection device； and a processor arrangement configured to: monitor the detected dose of medicament dialled into the attached injection device； detect a change in the state of the electromechanical switch arrangement； and change a display output of the supplemental device when a change in the state of the electromechanical switch arrangement is detected while a zero dose is dialled into the attached injection device.
