A supplemental device for attachment to an injection device, the supplemental device comprising: an electromechanical switch arrangement (110) having an open state and a closed state, the electromechanical switch arrangmenet (110) comprising a protrusion (118) configured to contact a surface (116, 117) of the injection device when a dose of zero units is dialled into the injection device, wherein the state of the switch arrangement (110) is configured to change when a dose dialled into the attached injection device is decreased from one unit to zero units； and a processor arrangement configured to: detect one or more changes in the state of the electromechanical switch arrangement (110)； and determine from the one or more state changes that a dose of zero units is dialled into the injection device.
