<P> Modified - release dosage is a mechanism that (in contrast to immediate - release dosage) delivers a drug with a delay after its administration (delayed - release dosage) or for a prolonged period of time (extended - release (ER, XR, XL) dosage) or to a specific target in the body (targeted - release dosage). </P> <P> Sustained - release dosage forms are dosage forms designed to release (liberate) a drug at a predetermined rate in order to maintain a constant drug concentration for a specific period of time with minimum side effects . This can be achieved through a variety of formulations, including liposomes and drug - polymer conjugates (an example being hydrogels). Sustained release's definition is more akin to a "controlled release" rather than "sustained". </P> <P> Extended - release dosage consists of sustained - release (SR) and controlled - release (CR) dosage . SR maintains drug release over a sustained period but not at a constant rate . CR maintains drug release over a sustained period at a nearly constant rate . </P>

What is meant by sustained or slow release