Tamoxifen is not only considered a very potent chemotherapeutic adjuvant for estrogen receptor positive breast cancers but also a very good chemo-preventive drug. Recently, there has been a rising amount of evidence for a nongenomic cytotoxicity of tamoxifen, even in estrogen receptor negative cells, which has greatly confounded researchers. Clinically, the side effects of tamoxifen can be very serious, ranging from liver steatosis to cirrhosis, tumorigenesis, or onset of porphyrias. Herein, we deciphered the nongenomic, mitochondrial cytotoxicity of tamoxifen in estrogen receptor positive MCF7 versus triple-negative MDA-MB-231 cells, employing the mitochondrial complex III quinoloxidizing-center inhibitor myxothiazol. We showed a role for hydroxyl-radical-mediated lipid peroxidation, catalyzed by iron, stemming from the redox interactions of tamoxifen quinoid metabolites with complex III, resulting in Fenton-capable reduced quinones. The role of tamoxifen semiquinone species in mitochondrial toxicity was also shown together with evidence of mitochondrial DNA damage. Tamoxifen caused an overall metabolic (respiratory and glycolytic) rate decrease in the Pasteur type MCF cells, while in the Warburg type MDA-MB-231 cells the respiratory rate was not significantly affected and the glycolytiv rate was significantly boosted. The nongenomic cytotoxicity of tamoxifens was hence associated with the metabolic phenotype and redox activity of the cells, as in the present paradigm of Pasteur MCF7s versus Warburg MDA-MB-231 cells. Our present findings call for caution in the use of the drugs, especially as a chemopreventive and/or in cases of iron overload diseases.