Multidrug resistance is a significant impediment to the successful treatment of neoplastic disorders. This type of treatment resistance has been studied in both cultivated tumor cell lines and clinical tumors. ABCB1 appears to play a critical role in these cells by serving as an energy-dependent efflux pump, removing numerous natural product medicines from the cell prior to their cytotoxic effects being exerted. ABCB1 (P-gp, MDR1) is an ABC transporter protein that is produced by the MDR1 gene. ABCB1 activity can significantly affect the pharmacokinetics of numerous chemotherapeutic drugs and thus the effectiveness of chemotherapy treatments of various diseases. On the other hand, simultaneous use of ABCB1 inhibitor drugs and ABCB1 substrate drugs may increase the blood concentration and intracellular concentration of the latter, which may increase the risk of unwanted drug-drug interactions and side-effects.