- The drug intestinal permeability (P-eff) measure has been widely used as one of the main factors governing both the rate and/or extent of drug absorption (F-abs) in humans following oral administration. In this communication we emphasize the complexity behind and the care that must be taken with this in vivo Puff measurement. Intestinal permeability, considering the whole of the human intestine, is more complex than generally recognized, and this can lead to misjudgment regarding F-abs and P-err in various settings, e.g. drug discovery, formulation design, drug development and regulation. Setting the adequate standard for the low/high permeability class boundary, the different experimental methods for the permeability measurement, and segmental-dependent permeability throughout the human intestine due to different mechanisms are some of the main points that are discussed. Overall, the use of jejunal P-eff as a surrogate for extent of absorption is sound and scientifically justified; a compound with high jejunal P-eff will have high F-abs, eliminating the risk for misclassification as a BCS class I drug. Much more care should be taken, however, when jejunal P-eff does not support a high-permeability classification; a thorough examination may reveal high-permeability after all, attributable to e.g. segmental-dependent permeability due to degree of ionization or transporter expression. In this situation, the use of multiple permeability experimental methods, including the use of metabolism, which except for huminal degradation requires absorption, is prudent and encouraged.