A self-admixing disposable hypodermic needle (admixing needle) includes a reservoir containing an additive, such as a buffering agent, to facilitate homogeneous mixing of the additive with an injection fluid, such as Lidocaine, contained in a cartridge held within a mechanically conjoined syringe. The admixing needle establishes a fluid communication pathway for hypodermically delivering the injection mixture to a patient preceding a potentially painful dental or medical procedure. Prior to removal, a delivery needle cap is pushed toward the syringe to dispense the additive into the cartridge. The delivery needle is then placed within the patient's tissues and the syringe is operated to force the injection mixture from the cartridge. Mixing surfaces on the male body (reservoir plunger) and the female body (reservoir) and other mechanical features homogeneously combine the injection fluid with the additive as the injection mixture travels through the admixing needle during the hypodermic injection.
