self - disabling or self - locking single - use syringe of the passive type , i . e ., one that is self - locked after first use of the syringe , in such a way that any attempted reuse is prevented , and is characterised in that it comprises at least two rings provided on the internal face of the barrel and having the general shape of a circular crown with a variable internal radius , and in that each of the longitudinal fins of the piston has a height which is different from that of at least one of the other fins , in such a way that the plunger can be inserted tightly through the space defined by the variable internal radius of the barrel rings . in this way resistance to the passage of the piston is minimised , thereby reducing the risk of causing bruises , and with a manufacturing cost substantially similar to that of conventional syringes .