This type of coating is used in prolonged release dosage forms where an initial dose of a drug substance is released into the body and the remaining (maintenance) doses are released at a constant rate to ensure that the desired therapeutic concentration is maintained. Dosage forms with continuous, evenly prolonged release provide a sustaining effect of the drug substance. They are more effective than forms with periodic release, as they provide a constant concentration of drug in the body at the therapeutic level without pronounced extremes, do not overload the body with excessively high concentrations. Dosage forms using sustained-release coatings avoid the need for repeated administration of the drug into the body, especially for those medicine that require maintaining a constant concentration in the blood for a long period of time. In addition, sustained release is used for high-dose medicine, the rapid release of which causes undesirable side effects (e.g., ulcer formation with rapid release of potassium chloride in the GI tract). Methods of creating drug products with sustained release of the drug substance include: increasing the particle size of the drug substance; placing the drug substance in a matrix; coating the drug substance or a dosage form containing the drug substance; and forming complexes of the drug substance with materials such as ion exchange resins.

Materials suitable for sustained-release coatings are: a mixture of waxes (e.g. beeswax) with glycerol monostearate, stearic acid, palmitic acid, glycerol monopalmitate and acetyl alcohol. They form polymers that dissolve slowly or degrade in the GI tract; shellac and zein, which do not degrade at low pH values; ethylcellulose, which forms a membrane around the dosage form that does not degrade in the GI tract. However, it is permeable to water, which dissolves the drug substance, as a result of which the latter diffuses into the GI tract; acrylic resins, the mechanism of action of which is similar to the action of ethylcellulose, i.e. there is a controlled diffusion of the drug substance through the coating; cellulose acetate (diacetate or triacetate); silicone elastomers. The development of sustained-release medicine using film coating technology can be accomplished by various methods, such as applying suitable film coatings to: dry granules (both heterogeneous and spheronized); microspheres containing the drug substance or coated with the drug substance; crystals of the drug substance; drug substance-ion exchange resin complexes; tablets (including minitabs).