A locking mechanism according to the preferred embodiment of the present invention is shown in use with a drug delivery system having a control module and a reservoir module. The reservoir and control modules are removably hinged together at one end and are held by the locking mechanism of the present invention at the other end. The locking mechanism includes a latch button rotatably and slideably mounted about a longitudinal axis in the control module. A locking bolt is further provided rotatably received on an eccentric portion of the latch button. The locking bolt is prevented from sliding on the latch button by a pin extending in the latch button and is also prevented from rotating with the latch button by its slideable receipt in tracks formed in the control module. The latch button has a first position wherein the locking bolt does not not engage with an inverted U-shaped engaging member of the reservoir module, a second position located longitudinally from the first position, and a third position located longitudinally from the first position and in a rotated position from the first and second positions allowing the locking bolt to engage with and place a force on the U-shaped engaging member of the reservoir module.