A controllable drug delivery device for delivering a liquid injectable drug, the device comprising a drug reservoir (22), a drug administration device (28), a displacement-generating battery (20), and a current-depletion circuit (85), wherein displacement generated by the battery as current is depleted from the battery by the current depletion circuit displaces a wall of the drug reservoir thereby causing the reservoir to expel liquid injectable drug contained therein via the drug administration device, and wherein the drug delivery device further includes a controller (78) that is responsive to a measured parameter indicative of displacement generated by the battery for applying a variable load (80) across the battery in order to provide a substantially constant-current depletion of the battery and thereby cause the drug delivery device to deliver a substantially constant drug delivery rate.