A patient controlled liquid drug administration device, comprising: a housing defining an axial cavity having inner and outer ends; a pump mounted near an inner end of the cavity, the pump defining a reservoir having a first wall fixed in the cavity and a second wall flexibly movable therein with respect to the first wall between reservoir-full and reservoir-empty positions; an inlet conduit having a first end connected to an inlet port of the pump and a second end connectable to a source of pressurized liquid drug; an outlet conduit having a first end connected to an outlet port of the pump and an opposite second end subcutaneously connectable to the patient; a clamp mounted in the cavity to move between a closed position constricting the outlet conduit and thereby preventing the flow of liquid through it, and an open position disengaged from the outlet conduit, thereby allowing the flow of liquid through it, the clamp being biased toward its closed position; a plunger engageable with the second wall of the reservoir and disposed for axial movement within the cavity between the first position in which the second wall is in the reservoir-full position and a second position in which the second wall is in the reservoir-empty position; a plunger-actuating button extending from the outer end of the cavity and disposed in the cavity for axial movement between extended and depressed positions, the button having a detent for latching the button in its depressed position and a ledge that engages the clamp and moves it to its open position when the button is moved to its depressed position; a spring under compression disposed between the plunger and the button; a spring catch mounted in the cavity that resiliently engages the detent in the button when the button is moved to its depressed position and holds the button in its depressed position against the force of the spring; and a catch release on the plunger that disengages the spring catch from the detent in the button when the plunger is moved to its second position; characterised in that the device comprises a lock having a first position preventing axial movement of the button when the button is in the extended position and the plunger is in the reservoir- empty position and a second position that permits axial movement when the button is in the extended position and the plunger is in the reservoir-full position. a housing defining an axial cavity having inner and outer ends; a pump mounted near an inner end of the cavity, the pump defining a reservoir having a first wall fixed in the cavity and a second wall flexibly movable therein with respect to the first wall between reservoir-full and reservoir-empty positions; an inlet conduit having a first end connected to an inlet port of the pump and a second end connectable to a source of pressurized liquid drug; an outlet conduit having a first end connected to an outlet port of the pump and an opposite second end subcutaneously connectable to the patient; a clamp mounted in the cavity to move between a closed position constricting the outlet conduit and thereby preventing the flow of liquid through it, and an open position disengaged from the outlet conduit, thereby allowing the flow of liquid through it, the clamp being biased toward its closed position; a plunger engageable with the second wall of the reservoir and disposed for axial movement within the cavity between the first position in which the second wall is in the reservoir-full position and a second position in which the second wall is in the reservoir-empty position; a plunger-actuating button extending from the outer end of the cavity and disposed in the cavity for axial movement between extended and depressed positions, the button having a detent for latching the button in its depressed position and a ledge that engages the clamp and moves it to its open position when the button is moved to its depressed position; a spring under compression disposed between the plunger and the button; a spring catch mounted in the cavity that resiliently engages the detent in the button when the button is moved to its depressed position and holds the button in its depressed position against the force of the spring; and a catch release on the plunger that disengages the spring catch from the detent in the button when the plunger is moved to its second position; characterised in that the device comprises a lock having a first position preventing axial movement of the button when the button is in the extended position and the plunger is in the reservoir- empty position and a second position that permits axial movement when the button is in the extended position and the plunger is in the reservoir-full position.