An injector having a two-stage injection mechanism, the injector comprising: an outer housing having a first end and a second end, the outer housing comprising a cylindrical hollow body having one or more helical cams disposed on an internal wall within the body at the first end; a cylindrical hollow rotor body having one or more cam bearing portions disposed at one end and at least one guide slot disposed in a circumferential wall, each guide slot comprising a first transverse portion extending into a second longitudinal portion, the rotary body being concentrically located within the body of the outer housing such that each cam bearing portion faces a corresponding helical cam; a plunger assembly comprising: a barrel for storing a fluid; a needle disposed at a first end of the barrel; and a plunger for dispensing the fluid through the needle; and wherein at least one guide projection is disposed on a circumferential wall of the plunger; the plunger assembly being at least partially locatable within the rotor body such that each guide projection locates in a corresponding guide slot and such that when the projections are located in the transverse portion the plunger is constrained from movement toward the first end of the barrel and when located in the longitudinal portion the plunger is free to move toward the first end of the barrel; in use the plunger assembly and rotor body being moveable within the outer housing between a retracted position wherein the needle is at least partially housed within the outer housing and an engaged position wherein the needle projects outwardly of a needle opening located at the first end of the outer housing, wherein the plunger assembly and rotor body are moveable to the engaged position responsive to a force aligned with the direction of movement of the plunger assembly being applied to a second end of the plunger assembly, and such that continued application of the force causes the one or more cam bearing portions to bear on the one or more helical cams, thereby causing the rotor body to rotate within the outer housing in turn causing the guide projections to align with longitudinal portion of the corresponding guide slot, thereby permitting movement of the plunger for dispensing the fluid. an outer housing having a first end and a second end, the outer housing comprising a cylindrical hollow body having one or more helical cams disposed on an internal wall within the body at the first end; a cylindrical hollow rotor body having one or more cam bearing portions disposed at one end and at least one guide slot disposed in a circumferential wall, each guide slot comprising a first transverse portion extending into a second longitudinal portion, the rotary body being concentrically located within the body of the outer housing such that each cam bearing portion faces a corresponding helical cam; a plunger assembly comprising: a barrel for storing a fluid; a needle disposed at a first end of the barrel; and a plunger for dispensing the fluid through the needle; and a barrel for storing a fluid; a needle disposed at a first end of the barrel; and a plunger for dispensing the fluid through the needle; and wherein at least one guide projection is disposed on a circumferential wall of the plunger; the plunger assembly being at least partially locatable within the rotor body such that each guide projection locates in a corresponding guide slot and such that when the projections are located in the transverse portion the plunger is constrained from movement toward the first end of the barrel and when located in the longitudinal portion the plunger is free to move toward the first end of the barrel; in use the plunger assembly and rotor body being moveable within the outer housing between a retracted position wherein the needle is at least partially housed within the outer housing and an engaged position wherein the needle projects outwardly of a needle opening located at the first end of the outer housing, wherein the plunger assembly and rotor body are moveable to the engaged position responsive to a force aligned with the direction of movement of the plunger assembly being applied to a second end of the plunger assembly, and such that continued application of the force causes the one or more cam bearing portions to bear on the one or more helical cams, thereby causing the rotor body to rotate within the outer housing in turn causing the guide projections to align with longitudinal portion of the corresponding guide slot, thereby permitting movement of the plunger for dispensing the fluid.