A drug delivery device is provided herein which includes a tubular body defining an opening. A drug cartridge is also provided having a distal end larger in diameter than the opening. With the drug cartridge being inserted into the tubular body in a use position, the distal end of the drug cartridge is located at least partially externally of the tubular body. The medical pen injector of the subject invention further includes an adaptor engageable with the body. The adaptor encompasses the distal end of the drug cartridge with the drug cartridge being in the use position. The adaptor includes features for mounting a needle assembly thereto. Advantageously, with the subject invention, a drug cartridge having a large distal end may be accommodated with a drug delivery device, e.g., a pen injector type device.
