The invention relates to a modular drug delivery system, comprising a reservoir unit and a control unit in combination with at least one further reservoir or control unit, whereby each combination of a reservoir unit and a control provides different capabilities. More specifically, the present invention provides a system for delivering a drug to a user, comprising a reservoir unit, a control unit, and at least one further reservoir unit or control unit, wherein each reservoir unit is adapted to receive at least one command from a control unit and perform an action in response thereto, and comprises a reservoir adapted to contain a liquid drug and includes, in a situation of use, associated outlet means. The system further comprises expelling means for expelling a drug out of the reservoir through the outlet means. Each control unit comprises control means providing one or more commands for controlling the operation of a reservoir unit, wherein each combination of a reservoir unit and a control unit is adapted to provide a system with different capabilities.