This study examined the comparative anticancer effects of flavonoids and diazepam in the cultured cancer cells. In the SNU-C4 colorectal and MDA-MB-231 breast adenocarcinoma cells, apigenin and fisetin, flavonoids, and diazepam inhibited cancer cell survival concentration and incubation-time dependently. Diazepam consistently inhibited FAS activity, a known anticancer mechanism of flavonoids, in a concentration dependent manner. Unlike diazepam, in highly aggressive breast MDA-MB-231 cells known to have a nuclear/perinuclear located PBR, PK11195, a specific PBR ligand enhanced the proliferation of cells, and the proliferative effect of PK11195 was reversed by an addition of lovastatin, a HMG-CoA reductase inhibitor. Diazepam- and flavonoids-induced cytotoxic activity in both cancer cell lines was not reduced by the addition of 5-fluorouracil (5-FU), a chemotherapeutic agent. Like flavonoids, diazepam inhibited the release of vascular endothelial growth factor (VEGF) and granulocyte-macrophage-colony stimulating factor (GM-CSF) into supernatants of cultured in the SNU-C4 and MDA-MB-231 cells. In conclusion, this study provided in vitro information on the safe use of sedative in oncologic patients.