Among hydroxycinnamic acids, caffeic, ferulic and p-coumaric acids have received considerable attention due to their biological activities. However, studies related to the biological activities of o-coumaric acid (OCA) are limited. In this regard, this study was designed to determine the chemopreventive potential of OCA in human breast cancer cells (MCF-7). The EC50 value of OCA was found to be 4.95 mM and was used throughout the study. Caspase-3 protein and mRNA levels increased by 59% and 72%. Similarly, protein and mRNA levels of Bax were increased by 115% and 152%. However, OCA treatment caused 48% and 35% decreases in Bcl-2 protein and mRNA levels. Cyclin D1 and cyclin dependent kinase-2 protein and mRNA levels decreased significantly. Moreover, p53 protein and mRNA levels increased by 178% and 245%, respectively. In addition to p53, PTEN protein and mRNA levels were induced. Although, CYP1A1, CYP1A2 and CY2E1 mRNA levels increased, CYP3A4 and CYP2C9 mRNA levels decreased in response to OCA treatment. These results suggest that OCA demonstrates anticarcinogenic activity on MCF-7 cells by activating multiple pathways. However, it also has high carcinogen activating and drug interaction potential. Therefore, serious precautions must be taken before using OCA.