In this study, phytochemical analysis and toxicity profile of leaf and flower extracts of Nerium oleander L. species collected from Giresun province (Turkey) were investigated. In phytochemical analyzes, the cardiac glycoside, alkaloid, saponin and tannin contents of the extracts were analyzed qualitatively and quantitatively. The physiological effects of extracts were determined by examining root elongation, weight gain and germination rates. Biochemical effects were determined by measuring the levels of malondialdehyde (MDA), glutathione (GSH), superoxide dismutase (SOD) and catalase (CAT), which are indicators of oxidative stress. Cytotoxic and genotoxic effects were investigated by mitotic index (MI), micronucleus (MN) and chromosomal abnormality (CA) tests. N. oleander leaf and flower extract applications caused significant decreases in the physiological parameters of Allium bulbs. SOD and CAT activity in root tip cells increased significantly after the application of leaf extract compared to the control group. Similar changes were observed in the application of flower extract, but these increases were found to be at a lower level compared to the increases induced by the leaf extract. An increase in MDA levels and a decrease in GSH levels were observed in groups treated with leaf and flower extracts. These changes show that the extracts cause deterioration in antioxidant/oxidant balance. It was determined that the extracts, which caused a decrease in MI rates and an increase in MN and CAs frequencies, showed the most prominent cytotoxic and genotoxic effects at 250 μg/mL doses. These toxic effects were associated with the phytochemical content of the extracts, and it was thought that cardiac glycosides and alkaloids, whose presence were detected in qualitative and quantitative analyzes, may play an important role in toxicity. Studies investigating the therapeutic properties of plants as well as their toxic effects are insufficient, which leads to the fact that plants exhibiting potential toxicity are not well known. Therefore, this study will lead many studies on the toxicity profile of the phytochemical contents of plants. Therefore, this study will draw attention to the investigation of the toxicity profile and phytochemical contents of plants and will lead to similar studies.