A drug delivery system (1) comprising first (35) and second reservoir units (30) with first and second identifiers (32), and a main unit (10) adapted to receive either of the first (35) and second reservoir units (30) in a mounted position. The main unit (10) comprises drug expelling means having dose setting means (11, 1, 14, 15, 19) comprising a rotational dose setting member allowing a user to set a desired dose size for the given drug, the dose size being set in fixed increments for a given state. The drug expelling means has a first state in which an increment corresponds to a first volume of drug, and a second state in which an increment corresponds to a second volume of drug, wherein the drug expelling means is set in the first state by means of the first identifier when a first reservoir unit is mounted, and the drug expelling means is set in the second state by means of the second identifier when a second reservoir unit is mounted.
