N-Nitroso metoprolol is a genotoxic derivative of metoprolol . It induces DNA fragmentation in rat hepatocytes when used at concentrations ranging from 0.1 to 1 mM. In vivo, N-nitroso metoprolol (1,000 mg/kg) induces micronucleation of rat hepatocytes but not erythrocytes from the bone marrow or spleen.