A needleless injector system including: a first injector assembly; a gas capsule disposed within the first injector assembly and containing pressurized gas, with a pierceable surface at the forward end thereof to provide an injection source; a gas capsule piercing member disposed within the first injector assembly, where the piercing member and the gas capsule are movable relative to each other; a second injector assembly containing a medical treatment fluid and having a forward facing injection orifice for injecting the medical treatment fluid into a patient; and a resilient seal mounted between the gas capsule and the first injector assembly to contain all pressurized gas within a forward end of the first injector assembly after pressurized gas is released from the gas capsule following piercing of the gas capsule, in order to cause an immediate increase in pressure in the medical treatment fluid contained within the second injector assembly.