An injection device 110 is described having a housing 112 that receives a syringe 114. 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 trigger 300 has a rest position, in which it engages the drive, retaining it in a position corresponding to the retracted position of the syringe 114, and a depressed position, in which it no longer causes the drive to be so retained. The sleeve 119 normally locks the trigger 300 in its rest position. However, depression of the sleeve 119 into the exit aperture 128, allows the trigger to be depressed. Thereafter, the trigger 300 is retained in its active position.
