A reservoir (7) for a drug delivery system (1) is provided, wherein the reservoir (7) comprises a cavity (30), the cavity (30) being filled with a dose of a fluid drug (8) such that the dose of the drug (8) occupies 95 % or more of the cavity (30), wherein the dose volume is less than 5 ml. Furthermore, a method for filling a reservoir (7) for a drug delivery system (1) is provided.