A drug delivery device (100; 200) comprising: a main housing; a syringe arranged in the main housing, wherein the syringe comprises a medicament; and a pneumatic power pack (120; 202) arranged in the main housing, wherein the pneumatic power pack (120; 202) comprises: (i) a pressurized gas source (122; 204) storing pressurized gas (123; 206); (ii) a valve (124; 208) for the pressurized gas source (122; 204); (iii) a sleeve (126; 210) having an inner wall (130; 211), wherein the sleeve (126; 210) is configured to receive pressurized gas (123; 206) released from the pressurized gas source (122; 204); and (iv) a plunger (128; 212) axially movable with respect to both the sleeve (126; 210) and the syringe, wherein the plunger (128; 212) is in sliding gas-tight engagement with the inner wall (130; 211) of the sleeve (126; 219), characterized in that the drug delivery device (100; 200) further comprises: a needle cover axially movable with respect to the main housing; wherein the needle cover is arranged to activate the valve by axial movement of the needle cover in a proximal direction, to release the pressurized gas (123; 206), and wherein the released pressurized gas (123; 206) flows into the sleeve (126; 210) and propels the plunger (128; 212) in a distal direction with respect to the sleeve (126; 210) and the syringe, so as to eject the medicament from the syringe. a main housing; a syringe arranged in the main housing, wherein the syringe comprises a medicament; and a pneumatic power pack (120; 202) arranged in the main housing, wherein the pneumatic power pack (120; 202) comprises: (i) a pressurized gas source (122; 204) storing pressurized gas (123; 206); (ii) a valve (124; 208) for the pressurized gas source (122; 204); (iii) a sleeve (126; 210) having an inner wall (130; 211), wherein the sleeve (126; 210) is configured to receive pressurized gas (123; 206) released from the pressurized gas source (122; 204); and (iv) a plunger (128; 212) axially movable with respect to both the sleeve (126; 210) and the syringe, wherein the plunger (128; 212) is in sliding gas-tight engagement with the inner wall (130; 211) of the sleeve (126; 219), characterized in that the drug delivery device (100; 200) further comprises: a needle cover axially movable with respect to the main housing; wherein the needle cover is arranged to activate the valve by axial movement of the needle cover in a proximal direction, to release the pressurized gas (123; 206), and wherein the released pressurized gas (123; 206) flows into the sleeve (126; 210) and propels the plunger (128; 212) in a distal direction with respect to the sleeve (126; 210) and the syringe, so as to eject the medicament from the syringe. (i) a pressurized gas source (122; 204) storing pressurized gas (123; 206); (ii) a valve (124; 208) for the pressurized gas source (122; 204); (iii) a sleeve (126; 210) having an inner wall (130; 211), wherein the sleeve (126; 210) is configured to receive pressurized gas (123; 206) released from the pressurized gas source (122; 204); and (iv) a plunger (128; 212) axially movable with respect to both the sleeve (126; 210) and the syringe, wherein the plunger (128; 212) is in sliding gas-tight engagement with the inner wall (130; 211) of the sleeve (126; 219), a needle cover axially movable with respect to the main housing; wherein the needle cover is arranged to activate the valve by axial movement of the needle cover in a proximal direction, to release the pressurized gas (123; 206), and wherein the released pressurized gas (123; 206) flows into the sleeve (126; 210) and propels the plunger (128; 212) in a distal direction with respect to the sleeve (126; 210) and the syringe, so as to eject the medicament from the syringe.