Abstract: A medical injection system adapted for a single-handed injection, the medical injection device comprising a gas compartment configured to contain a gas, the gas compartment comprising a piston, a reservoir intended to contain a drug composition, the reservoir having a proximal portion and a distal portion provided with a fluid exit configured to receive an injection needle, wherein the piston can move proximally between a distal position defining a first volume of the gas compartment to a proximal position defining a second volume of the gas compartment smaller than the first volume, and wherein the gas compartment is in gas communication with the reservoir in order to force the drug composition through the fluid exit, when the piston is moved proximally.