The invention relates to a drug delivery system for a patient comprising an insulin delivery device (100), a user interface (101), a controller (109) comprising instructions and a user definable threshold criterion (114; 116) of a residual availability of a drug provided in a reservoir (102) of the drug delivery system, wherein executing of the instructions by the controller (109) causes the controller (109) to control the drug delivery system to determine the amount of the drug that is currently available to the patient and compare the determined amount with the threshold criterion (114; 116), in response to the comparison, in case the determined amount is lower than or equal to the threshold criterion (114; 116), providing a signal to the patient via the user interface (101). The drug is insulin.