AcrB is a membrane protein and a multidrug efflux transporter. Although the recently solved X-ray crystal structures of AcrB provide a rough sketch of how drugs efflux, the pathway and mechanism have not been completely elucidated. In this study, a ligand-mapping method based on the 3D-RISM molecular theory of solvation, which we recently developed, is applied to AcrB in order to identify the drug efflux pathway. We use a fragment-based approach as a strategy to map chemical functionality on the internal surfaces. A few "multifunctional" ligand-binding sites, which recognize various types of functional groups, are detected inside the porter domain. Spatial links between the multifunctional sites indicate a probable multidrug efflux pathway. The frustrated environment of the protein cavity constructed of weak interactions between ligand and protein may be a mechanism for allowing smooth transportation through the protein. Guided diffusion appears to be the main mechanism for efflux.