A combination comprising a drug delivery device and an add-on device adapted to be releasably attached thereto, the drug delivery device comprising: - a housing, - a drug reservoir or means for receiving a drug reservoir, - drug expelling means comprising a moveable dose setting member allowing a user to set a dose amount of drug to be expelled, - an actuation member actuatable between a first and a second state, the first state allowing a dose amount to be set, the second state allowing the drug expelling means to expel a set dose, and - an indicator adapted to display the set dose amount during dose setting, and to display the remaining dose amount to be expelled during drug expelling, the add-on device comprising: - mounting means, - an electronically controlled display adapted to display a set dose amount, - a motion sensor assembly (381, 385) adapted to detect movement of the dose setting member when the add-on device is attached to the drug delivery device, - a camera device adapted to capture an image of the indicator, and - processor means, characterized in that - the processor means is adapted to: (i) control the display dynamically to show the set dose amount based on input from the motion sensor assembly, and (ii) determine an expelled dose amount based on input from the camera device. wherein: - an expelled dose amount is determined based on a first feature captured from a first image and a second feature captured from a second image, the first image being captured when a dose amount has been set and the second image being captured when a dose amount has been expelled, and - the first feature is used to determine a first position of the indicator, and the second feature is used to determine a second position of the indicator, the positional difference between the first and second positions being indicative of an expelled amount of drug. - a housing, - a drug reservoir or means for receiving a drug reservoir, - drug expelling means comprising a moveable dose setting member allowing a user to set a dose amount of drug to be expelled, - an actuation member actuatable between a first and a second state, the first state allowing a dose amount to be set, the second state allowing the drug expelling means to expel a set dose, and - an indicator adapted to display the set dose amount during dose setting, and to display the remaining dose amount to be expelled during drug expelling, - mounting means, - an electronically controlled display adapted to display a set dose amount, - a motion sensor assembly (381, 385) adapted to detect movement of the dose setting member when the add-on device is attached to the drug delivery device, - a camera device adapted to capture an image of the indicator, and - processor means, - the processor means is adapted to: (i) control the display dynamically to show the set dose amount based on input from the motion sensor assembly, and (ii) determine an expelled dose amount based on input from the camera device. (i) control the display dynamically to show the set dose amount based on input from the motion sensor assembly, and (ii) determine an expelled dose amount based on input from the camera device. - an expelled dose amount is determined based on a first feature captured from a first image and a second feature captured from a second image, the first image being captured when a dose amount has been set and the second image being captured when a dose amount has been expelled, and - the first feature is used to determine a first position of the indicator, and the second feature is used to determine a second position of the indicator, the positional difference between the first and second positions being indicative of an expelled amount of drug.