below the preferred embodiments of the invention , which are provided solely by way of illustration and do not limit the scope of the invention in any way , are described . the plunger is shown in detail in fig1 . in said figure it can be seen that the plunger comprises : an elongated body which defines a longitudinal axis , said elongated body comprising a plurality of longitudinal ribs ( 2 ) which extend substantially along the entire length of the plunger , and each one of the longitudinal ribs ( 2 ) of the plunger has a height which is constant along its entire length , but different from the height of the other ribs , said height being suitable for allowing insertion of the plunger ( 1 ) in a close - fitting manner through the space defined by the said variable - radius rings ( 8 , 9 ) of the barrel , the height of each one of said rings being reduced by the amount necessary for allowing insertion of the plunger in a close - fitting manner through the space defined by the rings of the barrel ; the ribs are preferably four in number since this is the number of ribs present in conventional syringes ; however , the invention is not limited by the number of ribs and any number of ribs is acceptable , provided that their number and height is variable in such a way that the plunger can be inserted in a close - fitting manner through the space defined by the barrel rings which in turn also have a variable inner radius ; the plunger also comprises a weakened zone which is situated preferably at the distal end of the plunger body and which may have any form , being preferably an orifice with a polygonal — generally triangular or square - shape ; a connecting piece ( 4 ) which joins the plunger to the piston ( 3 ); and a piston ( 3 ), usually a rubber part , the outer surface of which forms a fluid - tight connection with the inner surface of the barrel . fig2 shows an alternative embodiment of the plunger , comprising an additional tongue which projects slightly from the piston so that it increases the capacity for locking the plunger by means of the barrel rings . the syringe barrel is shown in detail in fig3 . this fig3 also shows views , in positions at 90 ° intervals ; of the barrel used in the single - use syringe according to the invention . in said figure it is possible to see the two rings with a substantially triangular cross - section and variable radius situated respectively on the inner top part ( 8 ) and inner bottom part ( 9 ) of the barrel . said rings may be completely closed , but preferably they are not closed and have a crescent shape ; the rings have a substantially triangular cross - section and the general shape of a circular rim with a variable inner radius so that the outer radius of each ring will always be the inner radius of the barrel , with which it makes integral contact , while the inner radius of each ring will be variable so as to minimise the total surface area of the ring so that it offers the least possible resistance to passage of the plunger , while causing breakage in the case of improper use , so that , preferably , they have a minimum inner radius on one of the sides of the barrel ( see 0 ° view ), an intermediate radius on the two sides situated at 90 ° from the first side ( see 90 ° and 270 ° views ) and an inner radius which has a maximum value or coincides with the outer radius ( in which case the thickness will be substantially zero ) on the side opposite to the first side ( see 180 ° view ). the general shape of the profile or cross - section of the rings is that of a wedge or triangle as explained above so that it is possible to insert the plunger inside the barrel , but not extract it easily . in an alternative embodiment of the invention , the said rings ( 8 , 9 ) of the barrel are not continuous , but are formed by more than one section with a discontinuous shape . the position of the rings on the inner surface of the barrel may vary so that it is possible to determine the volume amount of liquid which can be injected into the user before locking of the plunger occurs . therefore , although it is normally desirable that the top ring should be situated close to the distal end of the barrel relative to the needle with the aim of maximizing the useful volume , the bottom ring may be situated in a position close to the proximal end of the barrel relative to the needle , being separated from said end close to the needle by a greater or smaller distance , and in all cases preferably within the first third of the length of the barrel with respect to the needle . in this way , if the bottom ring is situated in a position on the barrel further away from the end close to the needle , locking of the plunger will occur when a smaller amount of liquid has been injected into the user . if , on the other hand , the bottom ring is situated in a position closer to the proximal end of the plunger relative to the needle , there is a greater useful volume of liquid which can be injected before locking of the plunger occurs , such that a drug - dependent user could take advantage of this fact in order to use the syringe again repeatedly without reaching the plunger locking position . preferably , the bottom ring will be situated in a position close to the end of the first third of the length of the barrel , with respect to the end close to the needle . the cap of the syringe , which is optional although present in a preferred embodiment of the invention , has a substantially conical shape with a widened side prolonged in the form of a claw , being in reality a protuberance of the cylinder segment type terminating in a sphere segment so that , upon insertion of the needle , the needle engages inside the cap in a direction perpendicular to the direction in which it is then inserted . namely , instead of inserting the needle in a single direction , as occurs with caps of the prior art , the user will perform engagement in a direction which is perpendicular to the shank of the cover and in this way never risks being pricked . once the user has performed engagement in a substantially horizontal direction , the shape of the claw of the cap prevents the needle from coming out of said cap at the top or on the sides and directs the needle towards the cone situated in the vertical plane downwards where it will be fixed in position , avoiding any risk of pricking one &# 39 ; s fingers . the syringe according to the invention comprises a plunger with the characteristics described above , which is inserted in a close - fitting manner inside a barrel having the characteristics also described above . the syringe will be provided with a needle which may be supplied together with the rest of the syringe or separately therefrom . when the plunger is inserted inside the barrel , the plunger must be situated so that the lowest - height rib coincides with the largest - radius side of the rings of the barrel ; the rib situated on the opposite side , which will be the rib with the smallest or no reduction in height , will coincide with the side where the rings have a substantially zero radius and the intermediate ribs will have an intermediate size matching the radius corresponding to that of the barrel ring at each respective point . in this way it will be possible to insert the plunger in a close - fitting manner inside the barrel , since the barrel rings are semi - rigid so as to allow the piston to pass through them in the direction towards the end of the barrel close to the needle , but prevent it from passing through in the return direction towards the end distant from the needle . once inserted , the outer surface of the piston ( 3 ) forms a substantially fluid - tight closure with the inner surface of the barrel . the syringe will be used by the user with the plunger inserted inside the barrel so that the piston has passed through the top ring of the barrel ; but not the bottom ring ( fig6 ). in this way , the user will be able to draw the corresponding liquid by introducing the syringe needle into said liquid and extracting the plunger to the height necessary for drawing off the desired quantity of liquid , the stroke of the plunger being limited by the top ring ( 9 ) of the barrel . 1 . initially the piston is situated between both crescent - shaped rings so that there is an air chamber between the needle and the piston , called “ dead space ”. 2 . after introducing the needle into the container with distilled water or similar liquid the plunger is moved in the direction away from the needle , drawing off the liquid which remains inside the barrel , together with an air zone . 3 . said liquid is inserted inside a container with the dry extract , without moving the plunger to the end of its stroke . 4 . the mixture is collected again once the dry extract has dissolved in the distilled water . 6 . the needle is inserted in the patient &# 39 ; s body and the liquid is injected into the patient &# 39 ; s body using the conventional method . 7 . when the piston reaches the end of its stroke , it is lightly retained in said position . the syringe and needle are extracted from the patient &# 39 ; s body . 8 . once extracted from the patient , the needle is inserted inside its cap and the plunger is pulled lightly so as to cause breakage thereof . should this not be performed voluntarily and an attempt be made to use the syringe again , upon attempting to draw off the distilled water the plunger will break owing to the action of the crescent - shaped ring which retains the piston . similarly , the syringe according to the invention cannot be easily tampered with by the user since , should the user attempt to tamper with it prior to initial use , upon trying to extract the plunger from the barrel , the piston ( 3 ) will come up against the top ring ( 8 ) of the barrel , preventing extraction thereof and also causing breakage of the plunger by means of the aforementioned weakened part should the user persist in the attempt to extract it . in this way the invention described provides a syringe which has the advantages mentioned , namely a simple design , ability to self - lock after initial use , guarantee of safety in medical and sanitary terms and capacity to resist tampering prior to use thereof .