A supplemental device for attachment to an injection device, the supplemental device comprising: 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 when a dose of zero units is dialled into the injection device, wherein the state of the switch arrangement 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 and determine from the one or more state changes that a dose of zero units is dialled into the injection device.
