Abstract: A controlled-release dosage form for a pharmaceutically-active agent comprises a core, in which the agent is dispersed, surrounded by a diffusion-limiting sleeve. The agent is released at a zero-order or approximately linear rate because the release rate of the agent will be governed entirely by erosion from exposed core surfaces; the surface area of which does not change substantially during the release process. Such a product may be made by co-extruding the core and sleeve material and slicing the extrudate.