P-gp inhibitor 20 (compound H27) is a low cytotoxicity P -glycoprotein ( P -gp ) inhibitor. P-gp inhibitor 20 inhibits the efflux function of P -gp in a dose-dependent manner (without affecting the expression of P -gp), thereby reversing the multidrug resistance (MDR) of MCF-7/ADR cells, with an IC 50 value of 46.6 nM. P-gp inhibitor 20 can be used for cancer research.