A mixing syringe having a first sealed chamber containing a powder (powder housing) and a second sealed chamber containing a liquid (liquid housing). When the user needs to inject a patient, he or she holds the mixing syringe approximately upright and depresses a plunger. This motion causes a piercer to pierce a foil seal separating the two chambers. The liquid then drops down into the powder housing. The liquid flows through a passage in a piston located in the powder housing, where it then comes in contact with the powder itself. As the user continues pressing the plunger downward, the piercer comes to rest within the piston and seals the passage through the piston, thereby locking the piercer and piston together. The device is then ready for an injection. As the plunger is further depressed, the piston expels the powder and liquid mixture through a needle.