Sustained release

Sustained-release (SR), extended-release (ER, XR, or XL), time-release or timed-release, controlled-release (CR), or continuous-release (CR or Contin) pills are tablets or capsules formulated to dissolve slowly and release a drug over time. The advantages of sustained-release tablets or capsules are that they can often be taken less frequently than instant-release formulations of the same drug, and that they keep steadier levels of the drug in the bloodstream. Sustained-release tablets are formulated so that the active ingredient is embedded in a matrix of insoluble substance (various: some acrylics, even chitin, these are often patented) so that the dissolving drug has to find its way out through the holes in the matrix. In some SR formulations the matrix physically swells up to form a gel, so that the drug has first to dissolve in matrix, then exit through the outer surface.

There are certain considerations for the formation of sustained release formulation:
 * If the active compound has a long half-life (over six hours), it is sustained on its own.
 * If the pharmacological activity of the active compound is not related to its blood levels, time releasing then has no purpose.
 * If the absorption of the active compound involves an active transport, the development of a time-release product may be problematic.
 * Finally, if the active compound has a short half-life, it would require a large amount to maintain a prolonged effective dose. In this case, a broad therapeutic window is necessary to avoid toxicity; otherwise, the risk is unwarranted and another mode of administration would be recommended.

The difference between controlled release and sustained release is that controlled release is a perfectly zero order release; that is, the drug releases over time irrespective of concentration. Sustained release implies slow release of the drug over a time period. It may or may not be controlled release.

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