This work describes the pharmacological activity of extracts of Papaver somniferum , a poppy species. P. somniferum products are still considered as a unique source of drug for many diseases. The present study was designed to determine antiproliferative and cytotoxic effects of P. somniferum extracts on HeLa (Human Cervix Carcinoma), HT29 (Human Colorectal Adenocarcinoma), C6 (Rat Brain Tumor Cells), and Vero (African Green Monkey Kidney) cell lines. Alkaloids-rich extracts of P. somniferum exhibited antiproliferative effects on various cancer cell lines, especially at high concentrations. We assessed the ability of extracts of P. somniferum to harm the membrane of the cells. Results indicated that P. somniferum extracts destroy cellular membrane in tumor cell lines at high concentrations. Remarkably, the LDH test results disclosed that cytotoxicity of P. somniferum on cells was low at mid concentrations. This may indicate its cytostatic potential. The results of this study support the efficacy of P. somniferum extracts as an anticancer agent.