Drug delivery injection pen with add-on dose capturing and display module

A drug delivery system as in 6, comprising a drug delivery unit and a data capture unit releasably attachable to each other, the drug delivery unit comprising:

• the drug expelling device,
• the setting device, and
• the actuation device,
• the data capture unit comprising:
• the electronically controlled capturing system, and
• the switch device.

A drug delivery system, comprising:

• drug expelling device for expelling an amount of drug from a reservoir, comprising:
• setting device allowing a user to set a dose amount of drug to be expelled, and
• actuation device for releasing or driving the drug expelling means to expel the set dose amount,
• an electronically controlled capturing system for capturing data representing a property related to an amount of drug expelled by the expelling means,
• an identifier associated with a member which moves during drug expelling and represents a property related to the amount of drug expelled from a reservoir by the expelling device, the capturing system being adapted to capture the identifier, and
• switch device for starting initialization of the capturing system, the switch device being actuated as the identifier starts to move when the drug expelling means has been released or starts to be driven by the actuation means,
• wherein dose data representing a property related to the amount of drug expelled from the reservoir by the expelling means is captured when the set dose amount has been expelled or expelling has stopped for predetermined amount of time.