OxyContin controlled-release tablets contain oxycodone HCl and are prepared using the AcroContin delivery system, which is an improvement upon the Contin system.[12] The AcroContin system provides a biphasic absorption profile due to both an immediate release of drug within one hour, which cannot be achieved using the Contin system, and a prolonged release over 12 hours. This system uses a dual-controlled matrix consisting of 2 hydrophobic polymers (ammonio methacrylate copolymer).[3] After ingestion, the GI fluid dissolves the tablet coating, exposing the hydrophobic acrylic matrix. An initial amount of oxycodone (~30-40% of the respective dose[23]) is immediately released upon contact with the GI fluid, which begins channeling into the pores of the tablet matrix. This immediate-release component is much greater than that of Avinza (~10% of the respective dose).[20] As GI fluid enters the tablet matrix, it dissolves the entrapped drug, which slowly diffuses out of the matrix pores. Oxycodone release from the tablets is pH independent, which allows for a uniform release throughout the GI tract. Since the matrix is hydrophobic and does not dissolve, patients should be advised that they may pass empty tablets or "ghosts" in the stool or via colostomy, and that this is not a concern since the active ingredient has already been released from the tablet.