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 .