A torsion spring driven injection device for delivering set doses of a liquid drug, comprising: - a housing (2, 3) storing a cartridge containing the liquid drug, - a needle cannula in liquid communication with the interior of the cartridge and having a distal tip shielded by a telescopically movable needle shield, which telescopically movable needle shield is axially movable from a first position in which the needle shield is extended to cover the distal tip of the needle cannula to a second position in which the needle shield is retracted to expose the distal tip of the needle cannula, - a piston rod having a threaded outer surface and a not circular cross-section, for driving the liquid drug out from the cartridge through the needle cannula, - a rotatable dose setting member by which a user can set the size of a dose to be injected, - a torsion spring which is strainable in response to the user rotating the dose setting member and releasable to drive the piston rod, and - a rotatable drive member mating the not circular cross-section of the piston rod or having an inner thread mating the outer thread of the piston rod, characterized by - a user operable injection button, activation of which releases the torsion spring, - a nut member releasable coupled to the housing (2, 3) and having an inner thread mating the outer thread of the piston rod or having a cross-section mating the not circular cross-section of the piston rod, such that the piston rod is moved axially in the housing (2, 3) upon rotation of the drive member relatively to the housing (2, 3), wherein the needle shield is configured to lock the nut member to the housing (2, 3) upon telescopically movement of the needle shield from the first extended position to the second retracted position, and unlock the nut member from the housing (2, 3) upon telescopically movement of the needle shield from the second retracted position to the first extended position. - a housing (2, 3) storing a cartridge containing the liquid drug, - a needle cannula in liquid communication with the interior of the cartridge and having a distal tip shielded by a telescopically movable needle shield, which telescopically movable needle shield is axially movable from a first position in which the needle shield is extended to cover the distal tip of the needle cannula to a second position in which the needle shield is retracted to expose the distal tip of the needle cannula, - a needle cannula in liquid communication with the interior of the cartridge and having a distal tip shielded by a telescopically movable needle shield, which telescopically movable needle shield is axially movable from a first position in which the needle shield is extended to cover the distal tip of the needle cannula to a second position in which the needle shield is retracted to expose the distal tip of the needle cannula, - a piston rod having a threaded outer surface and a not circular cross-section, for driving the liquid drug out from the cartridge through the needle cannula, - a rotatable dose setting member by which a user can set the size of a dose to be injected, - a torsion spring which is strainable in response to the user rotating the dose setting member and releasable to drive the piston rod, and - a rotatable drive member mating the not circular cross-section of the piston rod or having an inner thread mating the outer thread of the piston rod, characterized by - a user operable injection button, activation of which releases the torsion spring, - a nut member releasable coupled to the housing (2, 3) and having an inner thread mating the outer thread of the piston rod or having a cross-section mating the not circular cross-section of the piston rod, such that the piston rod is moved axially in the housing (2, 3) upon rotation of the drive member relatively to the housing (2, 3), wherein the needle shield is configured to lock the nut member to the housing (2, 3) upon telescopically movement of the needle shield from the first extended position to the second retracted position, and unlock the nut member from the housing (2, 3) upon telescopically movement of the needle shield from the second retracted position to the first extended position.