P-glycoprotein (P-gp) plays a critical role in drug oral bioavailability and modulation of this transporter can alter the safety and/or e?cacy pro?le of substrate drugs. Individual oral molecular excipients that inhibit P-gp function have been considered as a mechanism for improving drug absorption, but a systematic evaluation of the interaction of excipients with P-gp is critical for informed selection of optimal formulations of proprietary and generic drug products. A library of 123 oral molecular excipients was screened for their ability to inhibit P-gp in two orthogonal cell-based assays. b-Cyclodextrin and Light green SF yellowish were identi?ed as modest inhibitors of P- gp with IC50 values of 168 mM (95% CI, 118-251 mM) and 204 mM (95% CI, 5.9-1745 mM), respectively. The lack of e?ect of most of the tested excipients on P-gp transport provides a wide selection of excipients for inclusion in oral formulations with minimal risk of in?uencing the oral bioavailability of P-gp substrates.