A medical device for delivering a medicament to a patient, comprising: a syringe barrel having a forward end and a rear end and defining a reservoir within which the medicament may be contained; a needle cannula having a forward tip and a rearward end coupled to said forward end of said syringe barrel and being in fluid communication with said reservoir; a plunger rod having a first end with a stopper positioned in said reservoir and a second end having a thumb pad for receiving medicament delivery pressure for causing said stopper to move within said reservoir to cause the medicament to be expelled therefrom; a hollow shield body receiving said syringe barrel therein, said barrel being selectively movable within said shield body between a first position in which said forward tip of said needle cannula is exposed, and a second position in which said forward tip of said needle cannula is contained within said shield body; retainer means for releasably securing said syringe barrel in said first position; and an urging means arranged between a portion of said hollow shield body and a portion of said syringe barrel for urging said syringe barrel from said first position toward said second position; said thumb pad interacting with said retainer means upon movement of said stopper to a position proximate said barrel forward end to release said syringe barrel from said retainer means and enable said urging means to move said syringe barrel from said first position to said second position upon release of medicament delivery pressure from said thumb pad; said retainer means comprises a flange clip provided on said hollow shield body proximate a rear facing end thereof, said hollow shield body comprises a step having a rear facing surface for receiving an end of said urging means, and characterized in that said syringe barrel has a radial flange defined on an outer circumference thereof, said radial flange defining an abutment surface for said urging means and comprising locking means for locking said barrel in said second position. a syringe barrel having a forward end and a rear end and defining a reservoir within which the medicament may be contained; a needle cannula having a forward tip and a rearward end coupled to said forward end of said syringe barrel and being in fluid communication with said reservoir; a plunger rod having a first end with a stopper positioned in said reservoir and a second end having a thumb pad for receiving medicament delivery pressure for causing said stopper to move within said reservoir to cause the medicament to be expelled therefrom; a hollow shield body receiving said syringe barrel therein, said barrel being selectively movable within said shield body between a first position in which said forward tip of said needle cannula is exposed, and a second position in which said forward tip of said needle cannula is contained within said shield body; retainer means for releasably securing said syringe barrel in said first position; and an urging means arranged between a portion of said hollow shield body and a portion of said syringe barrel for urging said syringe barrel from said first position toward said second position; said thumb pad interacting with said retainer means upon movement of said stopper to a position proximate said barrel forward end to release said syringe barrel from said retainer means and enable said urging means to move said syringe barrel from said first position to said second position upon release of medicament delivery pressure from said thumb pad; said retainer means comprises a flange clip provided on said hollow shield body proximate a rear facing end thereof, said hollow shield body comprises a step having a rear facing surface for receiving an end of said urging means, and characterized in that said syringe barrel has a radial flange defined on an outer circumference thereof, said radial flange defining an abutment surface for said urging means and comprising locking means for locking said barrel in said second position.