Disclosed is a therapeutic fluid reservoir to be used with an ambulatory portable infusion device. The reservoir includes a pliable shell defining a varying inner volume that varies based on the volume of therapeutic fluid held inside the reservoir, the varying inner volume having a predetermined maximum volume for containing a corresponding volume of therapeutic fluid and an outlet port for communicating fluid from the reservoir to a patient, the varying inner volume decreasing upon therapeutic fluid being communicated through the outlet port. Upon the reservoir reaching the predetermined maximum volume when being filled with therapeutic fluid, any therapeutic fluid in excess of the predetermined maximum volume driven into the reservoir causes a substantially corresponding amount of therapeutic fluid to exit out of the reservoir via the outlet port and/or at least one other port provided in the pliable shell.
