Promethazine, a phenothiazine derivative, mainly used as an antihistamine and antiemetic agent, exhibited chemosensitizing activity in MCF7 human breast cancer drug resistant sublines when combined with vincristine or doxorubicin by modulating P-gp activity and decreasing the expression of MDR1 and MRP1 genes [131,132]. The gene discussed is PGP; the disease is breast carcinoma.