Objective:To investigate the molecular effects ofNerium oleanderleaf distillate on paclitaxel and vincristine resistant(MCF-7/Pac and MCF-7/Vinc)cells and sensitive(MCF-7/S)cell lines.Methods:Nerium oleander(N.oleander)leaf extract was obtained by hydrodistillation method.The toxicological effects ofN.oleanderdistillate,previously suggested as medicinal food supplement,on drug resistant cells were evaluated by XTT tests.MDR modulation potential of the plant material was evaluated by flow cytometry and fluorescent microscopy.Paclitaxel and vincristine were applied to the sublines in combination with N.oleanderdistillate.Results:Fractional inhibitory indices show thatN.oleanderdistillate did not increase the antiproliferative effects of anticancer drugs.N.oleandertreatment in to MCF-7/Pac and MCF-7/Vinc did not inhibit P-gp activity and MDR1 gene expression level.Conclusions:As a result it may be suggested that althoughN.oleanderdistillate has some medicinal effects as food supplement it may not be suitable as an MDR modulator for drug resistant breast cancer cells.