An osmotic pump system includes a capsule having at least one delivery port, a membrane plug retained at an open end of the capsule remote from the delivery port, the membrane plug providing a fluid-permeable barrier between an interior and an exterior of the capsule, and a removable imbibition rate reducer attachable to the capsule. The imbibition rate reducer comprises one or more flow controllers selected from the group consisting of an orifice having a selected size smaller than a surface area of the membrane plug and a membrane having a selected thickness, surface area, radial compression, and permeability. The imbibition rate reducer allows customizable delivery of medicaments.