The permeability of a substance across a membrane of given composition has been traditionally assumed to depend only on the properties of the molecule itself (charge, lipophilicity, molar weight, etc.). In textbooks, permeation is described as 1 st order kinetic process, proportional only to the concentration gradient of the permeating molecule alone. Experimental and computational permeation results have so far been interpreted in a way that assumed permeability to be a unary property. However, there is an increasing body of scientific literature pointing at potential drug-drug interactions in polypharmacy patients, many of whom are systematically over-or under-dosed due to significantly different bioavailability profiles when some drugs are prescribed in combination rather than alone . Interestingly, such interactions were reported even for drugs that target very different metabolic pathways and that should not, in theory, influence each other at the molecular target level. These phenomena could potentially be explained by considering permeability a binary (or higher order) property, i.e. by considering that the permeation rate of molecule A could also depend on the concentration of molecule B (or C, etc.). However, no direct experimental evidence for such collective permeation properties has been available so far, and in fact there was no method for reliably measuring co-permeation.