An auto-disable device for use with a sterile single use hypodermic syringe comprising: (a) a barrel adapter configured to attach to an outlet end of a barrel of the syringe, said attachment being external to the barrel; (b) a needle hub affixed to the barrel adapter and having a hypodermic needle affixed thereto; (c) the barrel adapter and the needle hub forming an assembly having a longitudinal axis, the assembly having a cavity with a passage for the flow of a liquid from an inlet of the assembly to the needle, the cavity having a section thereof that is within the needle hub and outside the barrel adaptor, the barrel adaptor having a cylindrical ring-retaining portion at an end thereof that engages the needle hub; (d) an elastically-deformable sealing ring in the cavity, the sealing ring having an annular base portion and an inwardly-extending sealing portion having an opening therein, the annular base portion being fitted into the ring-retaining portion of the barrel adaptor; (e) a sealing member in the cavity having a head and a shaft, the sealing member being moveable in the cavity in a longitudinal direction from a first position in which the head of the sealing member is on a side of the sealing portion of the ring facing the inlet of the assembly to a second position in which the head of the sealing member is on a side of the sealing portion of the ring facing the needle and within the section of the cavity that is within the needle hub; and (f) the sealing portion of the ring being configured to stop movement of the sealing member from the second position to the first position and to form a seal with the head, when the sealing member is in the second position, against a flow of liquid in the direction from the needle to the inlet of the assembly. (a) a barrel adapter configured to attach to an outlet end of a barrel of the syringe, said attachment being external to the barrel; (b) a needle hub affixed to the barrel adapter and having a hypodermic needle affixed thereto; (c) the barrel adapter and the needle hub forming an assembly having a longitudinal axis, the assembly having a cavity with a passage for the flow of a liquid from an inlet of the assembly to the needle, the cavity having a section thereof that is within the needle hub and outside the barrel adaptor, the barrel adaptor having a cylindrical ring-retaining portion at an end thereof that engages the needle hub; (d) an elastically-deformable sealing ring in the cavity, the sealing ring having an annular base portion and an inwardly-extending sealing portion having an opening therein, the annular base portion being fitted into the ring-retaining portion of the barrel adaptor; (e) a sealing member in the cavity having a head and a shaft, the sealing member being moveable in the cavity in a longitudinal direction from a first position in which the head of the sealing member is on a side of the sealing portion of the ring facing the inlet of the assembly to a second position in which the head of the sealing member is on a side of the sealing portion of the ring facing the needle and within the section of the cavity that is within the needle hub; and (f) the sealing portion of the ring being configured to stop movement of the sealing member from the second position to the first position and to form a seal with the head, when the sealing member is in the second position, against a flow of liquid in the direction from the needle to the inlet of the assembly.