A medication pen comprising: a pen body having a distal end and a proximal end, and having a first medication compartment and a second medication compartment; first and second dosing mechanisms (200, 202) and first and second dosage plungers engaged in the respective first and second medication compartments (32, 34); a movable housing received on the distal end of the pen body adapted to be received in first and second positions on the pen body; a pen needle assembly received on the distal end of the movable housing, said pen needle assembly comprising a patient end needle, a filler needle, and a collapsible mixing reservoir intermediate and accessed by said patient end needle and said filler needle; wherein the movable housing is movable between a first position in which the filler needle is in fluid communication with the first medication compartment and a second position in which the filler needle is in fluid communication with the second medication compartment; and wherein collapsing the mixing reservoir forces medication in the mixing reservoir through the patient end needle during an injection, said medication pen comprises a shaft connecting the movable housing and the pen body, said shaft sliding proximally and distally in the pen body from a proximal position in which the filler needle of the pen needle assembly is engaged in a medication compartment and a distal position in which the pen needle assembly rotates with respect to the pen body, such that a pen needle installed on the movable housing is moved from a first position in which the filler needle is in fluid communication with the first medication compartment and a second position in which the filler needle is in fluid communication with the second medication compartment. a pen body having a distal end and a proximal end, and having a first medication compartment and a second medication compartment; first and second dosing mechanisms (200, 202) and first and second dosage plungers engaged in the respective first and second medication compartments (32, 34); a movable housing received on the distal end of the pen body adapted to be received in first and second positions on the pen body; a pen needle assembly received on the distal end of the movable housing, said pen needle assembly comprising a patient end needle, a filler needle, and a collapsible mixing reservoir intermediate and accessed by said patient end needle and said filler needle; wherein the movable housing is movable between a first position in which the filler needle is in fluid communication with the first medication compartment and a second position in which the filler needle is in fluid communication with the second medication compartment; and wherein collapsing the mixing reservoir forces medication in the mixing reservoir through the patient end needle during an injection, said medication pen comprises a shaft connecting the movable housing and the pen body, said shaft sliding proximally and distally in the pen body from a proximal position in which the filler needle of the pen needle assembly is engaged in a medication compartment and a distal position in which the pen needle assembly rotates with respect to the pen body, such that a pen needle installed on the movable housing is moved from a first position in which the filler needle is in fluid communication with the first medication compartment and a second position in which the filler needle is in fluid communication with the second medication compartment.