In general, orally administered drugs are expected to display a uniform absorption rate throughout the small intestine. However, for some drugs this is not the case, and absorption windows may exist in specific (usually upper) parts of the intestine. Examples of such drugs are Gabapentin, Acyclovir and Ciprofloxacin. This kind of problematic absorption process may be due to absorption via some active transporting systems or active excretion via efflux pumps, such as P-gp. Physiological differences throughout the small intestine, that is irrelevant for most drugs but not for all, might also affect the absorption.
Drugs with a narrow absorption window require frequent dosing since traditional sustained release formulations would release most of the dose after the formulation had passed the absorption window. In order to obtain a prolonged effect, retention of the drug delivery systems (DDS) and release of the dose before the absorption window would be required.