An injection device for dispensing a dose of a substance, the device including a housing, a dispensing element which can be displaced relative to the housing and a locking element which can be coupled with the dispensing element during or after a substance dispensing operation so that after the dispensing element has effected a dispensing operation once, it can no longer be operated to dispense another dose, and a method of locking an injection device after a substance has been dispensed once, whereby a dispensing element operated to dispense the substance is locked or blocked before, during or after dispensing a set dose so that no more substance can be dispensed.