A

Most drugs are small, lipophilic molecules that move down their concentration gradient through the lipid bilayer of cells; this simple passive diffusion accounts for the bulk of drug passage across biological membranes, whereas carrier-mediated transport, active transport and endocytosis are relevant to a comparatively smaller subset of drugs [Sugano 2010 PMID 20671764; Cocucci 2017 PMID 27804130].