A needle-free injection device (30), comprises a housing a piston (80) disposed at least partially within the housing, the piston having a forward end and a rearward end, the piston being configured to advance relative to the housing from a rearward- most position to a forward-most position during delivery of an injection, so as to cause injectable fluid to be expelled out through an injection orifice a pressurized gas source configured to deliver pressurized gas to act upon the rearward end of the piston and a gas seal at the rearward end of the piston, the gas seal being configured to seal a variable-volume gas chamber defined within the housing between the rearward end of the piston and the pressurized gas source, where the needle-free injection device (30) is configured so that the gas seal is compromised upon advancement of the piston (80) to the forward-most position, so as to de-pressurize the variable-volume gas chamber after delivery of an injection.
