A system for mixing drug products and injecting, comprising: a syringe body defining a proximal opening at a proximal end thereof and a distal needle interface and a distal medicine port at a distal end thereof; proximal and distal stopper members (32, 36) disposed in the syringe body, forming a proximal drug chamber between the proximal and distal stopper members (32, 36) and a distal drug chamber between the distal stopper member and the distal end of the syringe body; a plunger member defining a plunger interior and configured to be manually manipulated to insert the proximal stopper member relative to the syringe body, the plunger member including a needle retention feature disposed in the plunger interior, an energy storage member disposed in the plunger interior, the energy storage member having a first stored mode and a second released mode, and an energy storage member latching member disposed in the plunger interior; and a needle hub assembly coupled to the distal needle interface of the syringe body, the needle hub assembly including a needle having a needle proximal end, a proximal mixing opening, and a distal mixing opening, such that the needle fluidly couples the proximal and distal drug chambers (252, 256), a hub, and a needle latching member configured to couple the needle to the hub, wherein the needle retention feature is configured to selectively couple the needle proximal end and the energy storage member such that inserting the distal stopper member to the distal end of the distal drug chamber causes the needle to be pulled proximally at least partially through the proximal and distal stopper members (32, 36), and wherein first and second sizes of the respective proximal and distal drug chambers (252, 256) can be modified by movement of the proximal and distal stopper members (32, 36) relative to the syringe body. a syringe body defining a proximal opening at a proximal end thereof and a distal needle interface and a distal medicine port at a distal end thereof; proximal and distal stopper members (32, 36) disposed in the syringe body, forming a proximal drug chamber between the proximal and distal stopper members (32, 36) and a distal drug chamber between the distal stopper member and the distal end of the syringe body; a plunger member defining a plunger interior and configured to be manually manipulated to insert the proximal stopper member relative to the syringe body, the plunger member including a needle retention feature disposed in the plunger interior, an energy storage member disposed in the plunger interior, the energy storage member having a first stored mode and a second released mode, and an energy storage member latching member disposed in the plunger interior; and a needle retention feature disposed in the plunger interior, an energy storage member disposed in the plunger interior, the energy storage member having a first stored mode and a second released mode, and an energy storage member latching member disposed in the plunger interior; and a needle hub assembly coupled to the distal needle interface of the syringe body, the needle hub assembly including a needle having a needle proximal end, a proximal mixing opening, and a distal mixing opening, such that the needle fluidly couples the proximal and distal drug chambers (252, 256), a hub, and a needle latching member configured to couple the needle to the hub, a needle having a needle proximal end, a proximal mixing opening, and a distal mixing opening, such that the needle fluidly couples the proximal and distal drug chambers (252, 256), a hub, and a needle latching member configured to couple the needle to the hub, wherein the needle retention feature is configured to selectively couple the needle proximal end and the energy storage member such that inserting the distal stopper member to the distal end of the distal drug chamber causes the needle to be pulled proximally at least partially through the proximal and distal stopper members (32, 36), and wherein first and second sizes of the respective proximal and distal drug chambers (252, 256) can be modified by movement of the proximal and distal stopper members (32, 36) relative to the syringe body.