A supplementary device for attachment to a drug delivery device, the supplementary device comprising: an attachment mechanism for coupling the supplementary device to the drug delivery device; an imaging arrangement configured to provide image data representing a captured image of one or more numbers present on a number sleeve of the drug delivery device at a location on the number sleeve corresponding to a currently set dose of medicament; and a processor arrangement configured to: receive the image data representing the captured image; perform optical character recognition on the image data to determine a quantity of the currently set dose of medicament; and cause display of the quantity of the currently set dose of medicament, characterised in that the processor arrangement is configured to: process the image data representing the captured image to identify one or more optical characteristics of the number sleeve at the location; determine from the determined optical characteristics of the number sleeve, and not from any numbers that are present on the number sleeve, whether the currently set dose of drug is a permitted dose; and in response to determining that the currently set dose of drug is not a permitted dose, cause an indication that the currently set dose of drug is not a permitted dose to be provided. an attachment mechanism for coupling the supplementary device to the drug delivery device; an imaging arrangement configured to provide image data representing a captured image of one or more numbers present on a number sleeve of the drug delivery device at a location on the number sleeve corresponding to a currently set dose of medicament; and a processor arrangement configured to: receive the image data representing the captured image; perform optical character recognition on the image data to determine a quantity of the currently set dose of medicament; and cause display of the quantity of the currently set dose of medicament, characterised in that the processor arrangement is configured to: process the image data representing the captured image to identify one or more optical characteristics of the number sleeve at the location; determine from the determined optical characteristics of the number sleeve, and not from any numbers that are present on the number sleeve, whether the currently set dose of drug is a permitted dose; and in response to determining that the currently set dose of drug is not a permitted dose, cause an indication that the currently set dose of drug is not a permitted dose to be provided. receive the image data representing the captured image; perform optical character recognition on the image data to determine a quantity of the currently set dose of medicament; and cause display of the quantity of the currently set dose of medicament, characterised in that the processor arrangement is configured to: process the image data representing the captured image to identify one or more optical characteristics of the number sleeve at the location; determine from the determined optical characteristics of the number sleeve, and not from any numbers that are present on the number sleeve, whether the currently set dose of drug is a permitted dose; and in response to determining that the currently set dose of drug is not a permitted dose, cause an indication that the currently set dose of drug is not a permitted dose to be provided. process the image data representing the captured image to identify one or more optical characteristics of the number sleeve at the location; determine from the determined optical characteristics of the number sleeve, and not from any numbers that are present on the number sleeve, whether the currently set dose of drug is a permitted dose; and in response to determining that the currently set dose of drug is not a permitted dose, cause an indication that the currently set dose of drug is not a permitted dose to be provided.