An injector, comprising: a substance container defining a substance chamber configured for containing a substance to be injected; an injection conduit configured for injecting the substance into a patient; a driver associated with the substance chamber for driving the substance through the injection conduit; a channel member having an interior surface defining a channel that has a first end open towards the interior of the channel member for receiving the substance from the substance chamber and a second end open towards the injection conduit, the channel extending between the first and second ends of the channel to fluidly communicate the substance container to the injection conduit from the first end to the second end of the channel (74, 100) for injecting the substance; a blocking stopper (70, 98) having a closed position for blocking flow of any substance from the substance container through the channel (74, 100) to the injection conduit, the blocking stopper (70, 98) being movable within the channel member with respect to the channel and to the channel ends to an open position for unblocking the flow of the substance from the substance chamber to the first end of the channel, through the channel around the blocking stopper (70, 98) to the second end, and to the injection conduit for injecting the substance, wherein the channel member extends between the first and second ends of the channel for bypassing a portion of the channel member in which the blocking stopper (70, 98) is disposed in the open position to fluidly communicate the substance container to the injection conduit from the first end to the second end for injecting the substance, and the channel comprises a guide protruding inwardly adjacent the channel to the interior surface for defining a circumferential side of the channel and for guiding and positioning the blocking stopper (70, 98) in the open position; and an energy source configured for driving the driver to both move the blocking stopper (70, 98) to the open position and inject the substance into the patient, characterised in that the injector further comprises a firing mechanism operably associated with the driver and operable by the user to cause the driver to drive the substance sufficiently to move the blocking stopper (70, 98) to the open position and also to inject the substance into the patient by a single action of the user on the firing mechanism. a substance container defining a substance chamber configured for containing a substance to be injected; an injection conduit configured for injecting the substance into a patient; a driver associated with the substance chamber for driving the substance through the injection conduit; a channel member having an interior surface defining a channel that has a first end open towards the interior of the channel member for receiving the substance from the substance chamber and a second end open towards the injection conduit, the channel extending between the first and second ends of the channel to fluidly communicate the substance container to the injection conduit from the first end to the second end of the channel (74, 100) for injecting the substance; a blocking stopper (70, 98) having a closed position for blocking flow of any substance from the substance container through the channel (74, 100) to the injection conduit, the blocking stopper (70, 98) being movable within the channel member with respect to the channel and to the channel ends to an open position for unblocking the flow of the substance from the substance chamber to the first end of the channel, through the channel around the blocking stopper (70, 98) to the second end, and to the injection conduit for injecting the substance, wherein the channel member extends between the first and second ends of the channel for bypassing a portion of the channel member in which the blocking stopper (70, 98) is disposed in the open position to fluidly communicate the substance container to the injection conduit from the first end to the second end for injecting the substance, and the channel comprises a guide protruding inwardly adjacent the channel to the interior surface for defining a circumferential side of the channel and for guiding and positioning the blocking stopper (70, 98) in the open position; and an energy source configured for driving the driver to both move the blocking stopper (70, 98) to the open position and inject the substance into the patient, characterised in that the injector further comprises a firing mechanism operably associated with the driver and operable by the user to cause the driver to drive the substance sufficiently to move the blocking stopper (70, 98) to the open position and also to inject the substance into the patient by a single action of the user on the firing mechanism.