Impact of the CYP2D6 genotype on the clinical effects of metoprolol: a prospective longitudinal study.

After administration of metoprolol, plasma concentrations of the drug are markedly higher in CYP2D6 poor metabolizers (PMs) than in non-PMs. In a prospective double-blind 3-month study, we investigated whether this translates into differences in metoprolol's effects after initiation of therapy. Despite administering equal doses to PMs and non-PMs, metoprolol plasma concentrations were 4.9-fold higher in the PM group. Metoprolol evoked significantly and persistently greater reductions in heart rate, diastolic blood pressure, and mean arterial pressure in PMs than in non-PMs. It appears, therefore, that the CYP2D6 genotype contributes to interindividual differences in metoprolol response.