A single-use syringe of the self-destructible or passive self-lockable type, namely in which self-locking thereof occurs after the syringe has been used a first time, independently of the user's will, is characterised in that it comprises at least two rings on the inner side of the barrel which have the general shape of a circular rim with a variable inner radius and in that each one of the longitudinal ribs of the plunger has a height which is different from that of at least one of the other ribs so that the plunger can be inserted in a close-fitting manner through the space defined by the variable inner radius of the barrel rings. In this way the resistance to passage of the plunger is miminised, reducing the risk of formation of haematomas, and with a manufacturing cost substantially similar to that of conventional syringes.
