An injection device is described having a housing that receives a syringe having a boot that covers its needle. The syringe is biased by a return spring from an extended position to a retracted position. A drive spring advances the syringe from its retracted position to its extended position. A return spring restores the syringe to its refracted position. A releasable locking mechanism retains the syringe in its refracted position. A sleeve can be depressed to release the locking mechanism. A cap covers the sleeve, thus preventing the locking mechanism from being released. When the cap is removed, it takes the boot with it and no longer prevents the locking mechanism from being released. Then, the locking mechanism can be released and the injection cycle begun.
