An opposite phenomenon -permeation retardation -remains rather unexplored although its biological and pharmacological implications can be just as important . The ability to suppress the permeation rate of specific compounds could, for example, enable previously rejected drugs, which were found to be too "leaky" and thus unsuitable for liposomal formulation , to be revisited. Not being aware of permeation enhancement or permeation suppression caused by a medicinal substance that was not a priori meant to do so, could be problematic especially in the context of the so-called polypharmacy patients, who are simultaneously prescribed many (typically five or more) medicines simultaneously.