OBJECTIVE Noscapine, a naturally occurring alkaloid obtained from opium poppy, is a microtubule-targeting agent. This study is aimed to investigate the effects of noscapine on human breast cancer cell lines by comparing them with those of tamoxifen and docetaxel. METHODS MCF-7 and MDA MB-23 cell lines were used to observe the effects of docetaxel, tamoxifen, and noscapine on cell proliferation. For each drug, cell blocks were prepared from cultured cells treated with IC50 dose of each drug and these were examined histologically. The expressions of Ki-67, Bcl-2, BAX, and cyclin-D1 were assessed immunohistochemically. RESULTS Although noscapine showed cytotoxic effects on both cell lines in a time and dose dependent manner, MDA-MB-231 cells were more susceptible to its effects. Noscapine inhibited MCF-7 and MDA-MB-231 cells proliferation in vitro with IC50 value of 29 µM and 69 µM, respectively, which was comparable with IC50 of tamoxifen (40 µM and 50 µM) and docetaxel (43 nM and 32 nM). Noscapine showed anti-proliferative effects by decreasing Ki-67, cyclin-D1 and apoptotic effects by increasing BAX/Bcl-2 ratio in both breast cancer cells. Its effect was comparable with tamoxifen and docetaxel. CONCLUSIONS Noscapine may be a good chemotherapeutic agent for the treatment of breast cancer, especially in estrogen receptor‑negative breast cancer (Tab. 2, Fig. 7, Ref. 40).