Abstract: In an injection syringe for use once only the piston rod (4) is connected to the piston (3) by claws (9) which upon pressing of the piston rod (4) against the piston (3) during injection break or become deformed to such an extent that it is no longer possible to retract the piston (3) for the purpose of refilling the syringe. The deformation is effected by means of a conical portion (13) on the piston, said portion by wedge effect pressing the claws (9) radially outwardly when the piston rod (4) is pressed against the piston (3). The rigidity and strength of the claws (9) is adapted to the resistance experienced during injection of a liquid so that the claws will certainly be broken or otherwise brought out of engagement with the piston (11). On the other hand the force required to push the piston forward in an empty syringe does not have to be sufficient to break the claws (9).