Multiple drug resistance is considered to be one of the leading reasons of failure of chemotherapy in cancer patients; therefore, oleanolic acid has potential as a chemotherapeutic agent itself in tumors with high expression of efflux transporters, such as MRP1 or P-gp, or as a co-adjuvant in the chemotherapy of P-gp/MRP1 expressing tumors [111]. Here, PGP is linked to cancer.