Abstract: A cartridge or injector for holding and mixing a medical product is provided herein including a tubular body having a cylindrical wall having first and second opposing ends. The wall includes inner and outer surfaces with the inner surface having a generally constant cross-section along at least a drug mixing area. A first stopper is slidably disposed within the wall between the first and second ends, the first stopper defining a liquid-tight seal with the inner surface. A second stopper is slidably disposed within the wall between the first stopper and the first end, the second stopper defining a liquid-tight seal with the inner surface. A flexible insert is disposed at or in proximity to the second end, the insert forming a liquid-tight seal with the wall, the insert being pierceable by a medical needle or other piercing instrument. A rigid retainer is fixed to the wall having an opening therein for providing access to the insert for piercing by a medical needle.