The spectrum of activities results from allelic variants of CYP2D6 and can lead to unwanted consequences upon assuming a standard dosing regimen based upon an average CYP2D6 activity. In fact, given this distribution of activities, upwards of 15% of patients will be left either essentially untreated or reacting to diverse drug complications without monitoring.
CYP2D6 metabolizes 25-30% of all clinically used drugs, including beta-blockers (e.g., metoprolol, carvedilol), antiarrhythmics, antidepressants (e.g., nortriptyline, fluoxetine), antipsychotics (e.g., haloperidol), and codeine. CYP2D6 has enormous genetic variability, with more than 75 allelic variants.