The present invention provides a needle-free injector for delivering a medicament under pressure into an animal or human, the injector having a charging mechanism comprising a threaded shaft/nut member combination. The injection device is of the type that includes an actuating device comprising: a gas tight chamber； a piston and rod assembly slidably received in the chamber and movable between a forward position and a rearward position； a gas charge in the chamber for urging the piston and rod assembly to the forward position； a mechanism for moving said piston and rod assembly against said gas charge into the rearward position； and a trigger for releasably retaining the piston and rod assembly in the rearward position. Also provided is a method and kit for using the device including the charging mechanism to administer a liquid through the skin of an animal or human.
