A retrospective cohort study later showed that potent CYP2D6-inhibiting antidepressants were associated with greater risk of precipitating serious hemodynamic events when combined with metoprolol than antidepressants with weak CYP2D6 inhibitory potential [25], and severe bradycardia and atrioventricular block have been reported in patients treated with these drug combinations [18, 26–30]. The gene discussed is CYP2D6; the disease is atrioventricular block.