Drug absorption through the digestive membranes occurs essentially by the paracellular route between the cells and through the tight junctions, and by the transcellular route via active or passive transfers across the membrane. Also, active transporters are able to pump substrates from the enterocytes back to the lumen via efflux proteins. These are able to slow and/or to reduce drug absorption, in particular, represent a source of variability by interactions with inducers or inhibitors such as pharmaceutical excipients. Other important factors are solubilization, drug metabolism, both in the lumen and in the enterocytes, gastric emptying and the interactions with food or between drugs.