volume in communication with the conduit or chamber the non return valves are respectively arranged at either side of the conduit so as to prevent reverse fluid flow from the conduit in to the cartridge 4 the rearward valve and to prevent reverse fluid flow from the needle to the conduit the forward valve the dosing mechanism 6 is actuated by being urged forwards by the actuation button 8 and may for example be mechanically assisted by a spring means during actuation the volume of the conduit is reduced creating a positive pressure in the conduit which acts urge the rearward non return valve closed and opens the forward valve and causes a metered dose to be expelled from the injector 1 the dosing mechanism 6 is biased rearwardly such that it returns to a primed position when pressure is released from the actuation button 8 as the dosing mechanism 6 moves rearwardly the volume of the conduit is increased resulting in a negative pressure which acts to acts urge the forward non return valve closed and opens the rearward valve this results in a metered dose being drawn from the cartridge 4 into the conduit thus the dosing mechanism is primed and ready to dispense a further metered dose upon the next actuation of the dosing mechanism 6 in accordance with the invention the injector 1 further comprises a two way non return mechanism 10 the non return mechanism 10 is arranged to both prevent the dose delivery mechanism from returning to the primed position until it has reached its end position i e it has been fully actuated and to prevent the dose delivery mechanism from being actuated until it has returned to the fully primed position as.