A composition for controlled release of an opioid from a pharmaceutical composition, the method comprises controlling the release of at least one opioid into an aqueous medium by erosion of at least one surface of a pharmaceutical composition comprising I) a matrix composition comprising a) polymer or a mixture of polymers, b) an opioid and, optionally, c) one or more pharmaceutically acceptable excipients, and (i) a coating. The matrix composition has a conus-like shape so the surface area exposed to the aqueous medium increases at least during initial erosion of the matrix composition, and the dissolution of the opioid—when tested in a Dissolution Test as described herein with or without application of sinkers-results in a zero order release of at least 80% of the opioid contained in the composition. Such compositions are especially suitable for controlled release of an opioid to obtain a delayed peak concentration and a prolonged therapeutically effective plasma concentration upon oral administration. Once or twice daily administration is possible. The matrix typically comprises PEO and the active substance is typically an opioid such as morphine or a glucuronide thereof.