A hand-held dual liquid medication injector have dual, bi-directional dosage metering mechanisms for permitting a variable dosage amount for each cartridge of liquid medication. The medications are mixed within a manifold having a valved mixing chamber and are injected via a single cannula. The mixing chamber is valved such that backflow of mixed or unmixed medications into the cartridges is prevented. An injection mechanism, independent of the metering mechanism, loads and injects the liquid medication. In one embodiment, downward movement of the injection mechanism during injection is translated into horizontal movement by the plunger mechanism. In another embodiment, a power-assisted plunger mechanism accomplishes injection.

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