A device for the automatic injection of doses of a drug compound is described. The device has a sliding sheath which, when depressed with its front end against the injection site, interacts with cam means to activate triggering of a plunger, thus controlling delivery of a drug dose. Plunger guide means are provided for controlling the triggering sequence and means for arming the device in the dose delivery condition. Automatic needle re-sheathing and resetting of a lock-out condition after each dose is delivered are also provided.