The present invention is directed to a dual chamber cartridge retainer mechanism suitable for medication delivery pens. The cartridge retainer mechanism is made of two chambers including an inner chamber tubular body surrounding the barrel of the cartridge and outer chamber supporting engaging inner chamber that defines and works as Cartridge retainer. The plunger mounting at surrounding the back side of the outer chamber includes a rim extending from its proximal end that is diametrically dimensioned to closely engage the outer chamber syringe piston.
