A medical device with a volume sensor is provided for administering liquid drug therapy to a user. The medical device comprises a flexible reservoir (101) for housing a supply of a liquid drug, a rigid structure (120) with a chamber (102) and a side hole (104) in fluid communication with the flexible reservoir, a plunger (105) mounted inside the chamber and configured to seal one side of the chamber, and a switch (106) implemented to indicate the position of the plunger. When the inside end of the plunger (105) passes the side hole (104), the switch (106) is activated to transmit a signal indicating liquid volume in the flexible reservoir (101).
