Controlled drug delivery systems have been referred to as delayed action, extended action, gradual action, prolonged release, repeat action, sustained release, depot, timed release systems.This is one of the therapeutic systems of that used locally to obtain a constant drug concentration.Controlled drug delivery occurs when a polymer, whether natural or synthetic, is judiciously combined with a drug or other active agent in such a way that the active agent is released from the material in a predesigned manner.The release of the active agent may be constant over a long period, it may be cyclic over a long period, or it may be triggered by the environment or other external events.the purpose behind controlling the drug delivery is to achieve more effective therapies while eliminating the potential for both under- and overdosing.Other advantages of using controlled-delivery systems can include the maintenance of drug levels within a desired range, the need for fewer administrations, optimal use of the drug in question, and increased patient compliance.