Here, we present original co-permeation experimental data obtained by means of a new liposome permeation assay on a sample of six commonly prescribed drugs. The principle of the method is shown in Fig. . Our data reveal both positive and negative interactions of copermeating molecules, providing the first direct evidence of collective permeation and partitioning behaviour that could have far-reaching consequences both for the prescription practices of existing drugs, and for the evaluation of new ones.