Needleless syringe provided with modular reservoir

The technical field of the invention is that of needleless pre-filled disposable syringes, operating with a gas generator, and used for intradermal, subcutaneous and intramuscular injections, of a liquid active principle for therapeutic use in human and veterinary medicine. The inventive needleless syringes use a reservoir (5) consisting of a tube (6) closed by an upstream plug-piston (7) and a downstream plug-piston (8) between which is contained the liquid active principle. Said needleless syringes are characterized in that they are capable of injecting a variable amount of liquid active principle (9) by adapting, through displacement of the downstream plug-piston (8) in the tube (6) the volume included between the two plug-pistons (7,8) to the amount of active principle (9) to be injected.