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

A drug delivery system, comprising:

• expelling device for expelling a dose of drug from a reservoir,
• an electronically controlled capturing system for capturing data representing a property related to the amount of drug expelled from a reservoir by the expelling means,
• switch device for starting initialization of the capturing system,
• a release member for releasing or driving the expelling means and adapted to be moved between:
• an initial position, and
• an actuated position in which the expelling means is released or driven,
• an actuation member adapted to be moved by a user between:
• an initial position,
• an intermediate position in which the switch means is actuated to start initialization of the capturing system, and
• an actuated position in which the release member is actuated,
• actuation biasing means providing a biasing force against movement of the actuation member between its initial and intermediate position,
• whereby the capturing system is allowed to initialize during the actuation member's movement between the intermediate position and the actuated position.