Medicinal plant extracts have recently attracted attention of modern medical science research due to their non-lethal activity. Currently, up to 50% of the world drugs including chemotherapeutic drugs such as taxol and camptothecin are derived from natural products. Euphorbia tirucalli has a long history of usage as traditional medicine in Africa and has been widely used in the treatment of different cancers. In this study, we explore the medical properties of E. tirucalli extracts in breast cancer development. To achieve this, stems of E. tirucalli were dried, crushed and extracted with butanol, hexane or methanol (based on 1 g of dry substance in 10 mL of a solvent). The dried extracts were re-dissolved in DMSO and investigated. Composition of each extract was analyzed using liquid chromatography-mass spectroscopy (LC-MS). Extracts were found to contain different types of secondary metabolites mainly terpenes and flavonoids. Breast cancer cell lines (MCF-7 and MDA-MB 231) were treated with various concentrations of the extracts for up to 48 h. Cell viability, cell cycle, apoptosis and gene expression were analysed. In cells, extracts were found to inhibit cell proliferation in a concentration and cell type dependent manner. Analysis of the cause of antiproliferation revealed that most cells were arrested at the G0/G1 phase by p21 overexpression. In general, most pro-apoptotic genes like Bax and caspase-8 were significantly up-regulated in cells treated with plant extracts. These results suggest that the extracts might induce cell cycle arrest at G0/G1 with p21 attributing to this molecular mechanism.