An attachable needle storage device is provided with a main body and a rotatable sleeve and a plurality of new needles stored within the sleeve, a leaf spring to direct each needle into a circumferential position of the pen body when rotated, and a ramp to move the needle into an axial position of the pen body when retracted or pulled rearward by the user. The attachable needle storage device is configured to attach to a pen device in a manner substantially the same as a conventional needle hub. In doing so, the rotation of the sleeve can be used to rotate a new needle onto a shuttle, and the retraction of the device can be used to cause a ramp to engage the needle and needle hub, moving the needle toward the axial position of the pen body and the septum. Once under the septum, the user continues pulling the device rearward, such that the needle is then moved rearward, pierces the septum of the medicament cartridge and exposes the distal end of the needle for injection. After use, a needle shield is used to unlock a compressed spring, which is configured to pull the needle from the septum, and return the device to the transport state, after which the previous steps can be repeated.