SYRINGE

The present invention relates to syringes for apportioning set doses from an ampoule has a dose setting and injection mechanism comprising a dose setting member which may be moved in one direction to set a dose and in the opposite direction to inject the set dose. A piston advancing mechanism comprises a piston rod and a piston rod drive, and the dose setting and injection mechanism is coupled to the piston advancing mechanism through a unidirectional coupling transmitting only movement of the dose setting member in the dose injecting direction to the piston rod drive. Operation of an air shot button enables influence on the piston advancing mechanism to advance the piston to expel an adjustable fixed volume. The air shot button is either coupled to the piston rod drive through a unidirectional coupling through which the piston rod is advanced when the air shot button is pressed, or it moves the end stop for an injection button so that this button may be pressed beyond its normal depressed end position.