A drug delivery system (10) has a positive pressure drug reservoir (15). A pump (19) is in fluid communication with the reservoir (15) via a first fluid passageway (18). The outlet (19b) of the pump (19) is in fluid communication with a first regulator (21). The regulator (21) includes a housing (24) that has a first subchamber (24a) and a second subchamber (24b). The second subchamber (24b) is divided into a lower chamber (28a) and a middle chamber (28b) by a diaphragm (28). A third fluid passageway (32) is positioned between the lower chamber (28a) and the drug reservoir (15), whereby accidental flow from the reservoir (15) to an outlet (21b) is prevented.