A housing (212) receives a syringe and includes a return spring (226) for biasing the syringe from an extended position in which its needle (218) extends from the housing to a retracted position in which it does not. A drive spring (230) acts on a first drive element (232) and a second drive element (234) acts upon the syringe to advance it from its retracted position to its extended position and discharge its contents through the needle. The first drive element is capable of movement relative to the second once a nominal coupling position has been reached. A release mechanism is activated when the first drive element (234) is further advanced to a nominal release position, to release the syringe (214) from the action of the drive spring, whereupon the return spring restores the syringe to its retracted position.
