Terifrac Pen is a reusable injection pen device, not a prefilled pen. It comes with the pen device, BD pen needles, and a user manual. The pen is used with a separate Terifrac cartridge. How long it lasts depends on the cartridge strength and your prescribed dose. The reusable pen can be used with multiple cartridges, and the cartridge lifespan varies based on your treatment regimen. Please follow your doctor's instructions and the product manual for proper use.