Factors affecting passive drug absorption

The lipid-aqueous partition coefficient of a drug molecule affects its absorption by passive diffusion. In general, octanol/pH 7.4 buffer partition coefficients in the 1-2 pH range are sufficient for absorption across lipoidal membranes. However, the absence of a strict relationship between the partition coefficient of a molecule and its ability to be absorbed is due to the complex nature of the absorption process. Absorption across membranes can be affected by several diverse factors that may include the ionic and/or polar characteristics of the drug and/or membrane as well as the site and capacity of carrier-mediated absorption or efflux systems. 

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