Preparations of Echinacea purpurea (E. purpurea) are widely used for the management of upper respiratory infections, influenza, and common cold, often in combination with other conventional drugs. However, the potential of phytochemical constituents of E. purpurea to cause herb−drug interactions via ABCB1 and ABCG2 efflux transporters remains elusive. The purpose of this study was to investigate ...