Cartridge and rod for axially loading medication pump

The present invention concerns a medication pump, a cartridge for use in a medication pump, a pump cap for use with a medication pump, components of the cartridge and methods of use of these devices. A cartridge includes a plunger having a tab that projects from an interior wall of the plunger. The tab can be used to lock the plunger into engagement with a drive rod of the pump. The drive rod defines a channel for receiving the tab of the plunger, where the channel includes an axial portion disposed parallel to an axis of the drive rod and a locking portion disposed in a circumferential direction around an outer surface of the drive rod. A similar channel structure can be provided on a removable fill rod used while filling the cartridge. The cartridge axially inserted into a cartridge chamber and then is rotated into engagement with the drive rod. The cartridge may also include axial guides at one end that interface with a guides on pump cap. When the pump cap is rotated into engagement with the pump, the interacting guides cause the cartridge to be rotated into engagement with the drive rod.