Multidrug resistance is a serious problem in modern medicine and the reason for the failure of various therapies. A particularly important problem is the occurrence of multidrug resistance in cancer therapies which affects many cancer patients. Observations on the effect of metformin—a well-known hypoglycemic drug used in the treatment of type 2 diabetes—on cancer cells indicate the possibility of an interaction of this substance with drugs already used and, as a result, an increase in the sensitivity of cancer cells to cytostatics. The aim of this study was to evaluate the effect of metformin on the occurrence of multidrug resistance of breast cancer cells. The MCF-7-sensitive cell line and the MCF-7/DX cytostatic-resistant cell line were used for this study. WST-1 and LDH assays were used to evaluate the effects of metformin and doxorubicin on cell proliferation and viability. The effect of metformin on increasing the sensitivity of MCF-7 and MCF-7/DX cells to doxorubicin was evaluated in an MDR test. The participation of metformin in increasing the sensitivity of resistant cells to the effect of the cytostatic (doxorubicin) has been demonstrated.