Injection device for administering a liquid comprising a pre-filled syringe and a safety mechanism for providing needle safety for an injection needle (3.2) of the pre-filled syringe, wherein the pre-filled syringe comprises - a barrel (3.1) containing the liquid, - a stopper (3.5) translatably disposed within the barrel (3.1), - the injection needle (3.2) attached to a distal end of the barrel (3.1) and wherein the safety mechanism comprises - a substantially cylindrical housing adapted to contain the pre-filled syringe, - a plunger connectable to the stopper (3.5) of the pre-filled syringe and adapted to be translated in a distal direction to expel the liquid through the injection needle (3.2), - an energizable biasing means arranged between the housing and the pre-filled syringe, wherein the energized biasing means is capable of biasing the pre-filled syringe with respect to the housing in a proximal direction and - a retaining mechanism for retaining the pre-filled syringe with respect to the housing in a first retracted position, in an advanced position and in a second retracted position, wherein the injection needle (3.2) is covered by the housing in the first and second retracted positions (R1, R2) and wherein the injection needle (3.2) projects distally from the housing in the advanced position, wherein the biasing means is arranged in a non-energized state when the pre-filled syringe is in the first retracted position and wherein the biasing means is adapted to be energized upon translation of the pre-filled syringe from the first retracted position to the advanced position so as to bias the syringe from the advanced position towards the second retracted position, and wherein a boot is detachably connected to the distal end of the barrel (3.1) that covers the injection needle (3.2) before an injection and provides a means for manually translating the pre-filled syringe from the first retracted position to the advanced position. - a barrel (3.1) containing the liquid, - a stopper (3.5) translatably disposed within the barrel (3.1), - the injection needle (3.2) attached to a distal end of the barrel (3.1) and wherein the safety mechanism comprises - a substantially cylindrical housing adapted to contain the pre-filled syringe, - a plunger connectable to the stopper (3.5) of the pre-filled syringe and adapted to be translated in a distal direction to expel the liquid through the injection needle (3.2), - an energizable biasing means arranged between the housing and the pre-filled syringe, wherein the energized biasing means is capable of biasing the pre-filled syringe with respect to the housing in a proximal direction and - a retaining mechanism for retaining the pre-filled syringe with respect to the housing in a first retracted position, in an advanced position and in a second retracted position, wherein the injection needle (3.2) is covered by the housing in the first and second retracted positions (R1, R2) and wherein the injection needle (3.2) projects distally from the housing in the advanced position, - a substantially cylindrical housing adapted to contain the pre-filled syringe, - a plunger connectable to the stopper (3.5) of the pre-filled syringe and adapted to be translated in a distal direction to expel the liquid through the injection needle (3.2), - an energizable biasing means arranged between the housing and the pre-filled syringe, wherein the energized biasing means is capable of biasing the pre-filled syringe with respect to the housing in a proximal direction and - a retaining mechanism for retaining the pre-filled syringe with respect to the housing in a first retracted position, in an advanced position and in a second retracted position, wherein the injection needle (3.2) is covered by the housing in the first and second retracted positions (R1, R2) and wherein the injection needle (3.2) projects distally from the housing in the advanced position,