An injection device 110 is described having a housing 112 that receives a syringe 114 having a sealed boot 123 that covers its needle 118. The syringe 114 is biased by a return spring 126 from an extended position in which the needle 118 extends from the housing 112 through an exit aperture 128 to a retracted position in which it does not. A drive spring 130 acts via a drive to advance the syringe 114 from its retracted position to its extended position and discharge its contents through the needle 118 and a return spring 126, brought into play when the drive has reached a nominal return position, restores the syringe 114 to its retracted position. A releasable locking mechanism retains the syringe 114 in its retracted position. A sleeve 119 projects from the exit aperture 128 and can be depressed to release the locking mechanism. A removable threaded cap 111 closes the housing 112, covers the exit aperture 128 and the sleeve 119, thus preventing the locking mechanism from being released, and engages the boot 123 on the syringe 114. When the cap 111 is removed, it takes the boot 123 with it, no longer closes the exit aperture 128 and no longer prevents the locking mechanism from being released. Then, the locking mechanism can be released and the injection cycle begun.
