Bezielle is an orally administered aqueous extract ofScutellaria barbatafor treatment of advanced and metastatic breast cancer. Phase I trials showed promising tolerability and efficacy. In our study, we used a combined proteomic–metabolomic approach to investigate the molecular pathways affected by Bezielle in ER-positive BT474 and ER-negative SKBR3 cell lines. In both, Bezielle inhibited cell proliferation, induced cell death and G2 cycle arrest by regulating the mediator proteins Jab1, p27Kip1 and p21Cip1. In addition, it stimulated reactive oxygen species production, hyperactivation of PARP and inhibition of glycolysis. Bezielle's ability to induce oxidative stress was associated with the changes in expression of redox potential maintaining enzymes: glutathione- and thioredoxin-related proteins and peroxiredoxins. In regards to cell metabolism, decreased expression of α-enolase was associated with a reduction ofde novo13C-lactate formation. Reduced Krebs cycle activity as evidenced by the reduced expression of α-ketoglutarate dehydrogenase and succinyl-CoA synthetase led to decreased intracellular succinate concentrations. By inhibiting glucose metabolism, cells reacted by lowering the expression of glucose transporters and resulting in decreased intracellular glucose concentration. Decreased expression of fatty acid synthase and reduced concentration of phosphocholine indicated considerable changes in phospholipid metabolism. Ultimately, by inhibiting the major energy-producing pathways, Bezielle caused depletion of ATP and NAD(H). Both cell lines were responsive, thus suggesting that Bezielle has the potential to be effective against ER-negative breast cancers. In conclusion, Bezielle's cytotoxicity toward cancer cells is primarily based on inhibition of metabolic pathways that are preferentially activated in tumor cells thus explaining its specificity for cancer cells.