An injection device has a main drive module which can be screwed into the back of a syringe housing to prepare the injector ready for use. The drive module has a firing button and a drive plunger released for forward movement by pressing the firing button. The syringe housing slideably receives a syringe holder whose forward movement is initially prevented by internally directed barbs on a cylindrical latching sleeve. The latching sleeve is acted upon by a release ring on the drive module such that as the drive module is screwed onto the syringe housing, the release ring splays the cylindrical shell to release the syringe carrier for forward movement. The barbs also act as a latch to capture the syringe carrier in its rearward position when the drive module is unscrewed from the syringe housing, which returns the barbs to their initial position before the syringe holder returns to its initial position.
