as is conventional at the end of this movement as shown in fig6 an outer tapered surface 70 formed at the distal end of stem extension 64 frictionally engages a complementary inner tapered surface 72 of carrier 24 the frictional engagement between surfaces 70 72 is sufficient so that when the user subsequently pulls on thumb pad 62 in a proximal direction the frictional engagement overcomes the resistance of retaining shoulders 48 thus pulling carrier 24 and cartridge needle unit 6 therewith in the proximal direction until boss 32 engages proximal locking region 50 at this point carrier 24 and cartridge needle unit 6 are effectively locked within body 26 with tip 58 sheltered by side wall 34 of the body as shown in fig7 to prevent the inadvertent or unauthorized removal of stem assembly 28 from body 26 body 26 has a pair of spring tabs 74 with radially inwardly directed teeth 76 at open proximal end 40 of body 26 teeth 76 are positioned to engage a tapered locking region 78 formed in stem extension 64 region 78 is defined in part by a radially extending shoulder 80 which teeth 76 engage in the event of attempted complete removal of stem assembly 28 from body 26 in the preferred embodiment the pre use and safe disposal positions of stem assembly 28 relative to body 26 are the same proximal position as shown in fig3 and 7 however stem assembly 28 could assume different proximal positions relative to body 26 if desired in use syringe 2 in the pre use condition in fig1 and 3 is selected and safety sheath 20 is removed the injection is then given in a conventional manner by pressing on thumb pad 62 while grasping.