A kind of syringe include a syringe,One push rod and a syringe needle external member,Wherein there is an opening in syringe one end,It can be slided in what syringe in order to being airtightly inserted into for push rod,And the syringe other end has a top surface,An outlet neck is integrally convexly equipped on what top surface,And the lateral border of syringe needle external member set what outlet neck； Outlet neck connect place with top surface and is defined as a linkage section,Lateral margin is equipped with an adapter section within linkage section,It thereby enables the thickness of linkage section compare what outlet neck and top surface and prejudges area in weakness relatively to be formed； The inner end of push rod is convexly equipped with an engaging portion,Thereby be combined with each other with the adapter section of lateral margin in linkage section positioning in a rotative pattern,And by excessive screwing and the torsion and thrust of advance enable weak area of prejudging be destroyed and be broken,Separate outlet neck with syringe,And syringe needle external member can be retracted in syringe.一種注射器包括有一針筒、一推桿及一針頭套件，其中針筒一端有一開口，俾供推桿氣密地插入而可於針筒內滑動，而針筒另一端具有一頂面，於頂面上一體凸設有一出口頸，而針頭套件套於出口頸之外側緣；出口頸與頂面連接之處界定為一連接段，連接段之內側緣環設有一結合段，藉此令連接段之厚度相較於出口頸及頂面呈相對薄弱以形成預斷區；推桿之內端凸設有一結合部，藉此與連接段內側緣之結合段以旋轉方式相互結合定位，且藉由過度旋緊且前進之扭力及推力令薄弱的預斷區遭到破壞而斷裂，使出口頸與針筒分離，而可將針頭套件拉回針筒內。
